What is sleep?

From a behavioral stand-point, sleep is a reversible behavioral state of perceptual disengagement from, and unresponsiveness to, the environment (Carskadon & Dement, 2011).

Within sleep, two broadly-defined states exist: rapid-eye movement (REM) sleep and non-REM (NREM) sleep. Sleep is normally entered through NREM sleep. NREM sleep is traditionally made up of sleep stages 1,2,3, and 4, which are defined by changes in brain activity (measured using Electroencephalograpy [EEG]) and generally reflect increasing sleep depth as one progresses through to stage 4. The brain alternates between NREM and REM sleep in what is a termed a sleep cycle.

Sleep cycles tend to last approximately 90 minutes and good sleepers generally go through 4-5 sleep cycles in a given night. The composition of sleep cycles change throughout the night, with the first third of the night tending to be characterized by greater amounts of slow-wave (deep) sleep (stages 3&4), whereas REM sleep tends to dominate in the latter third of the night, coinciding with a reduction in core-body temperature. Arousal threshold (the noise required to induce an awakening) also differs according to sleep stage. It is known, for example, that subjects are hardest to wake from slow-wave sleep, reflecting greater intensity and depth of sleep.

In NREM sleep, thought content is usually absent or, if present, fragmented but logical. In contrast, REM sleep tends to be characterized by vivid and often bizarre, illogical thought content, which is internally generated. REM sleep is the stage of sleep most often associated with dreaming, and where some regional brain areas are as metabolically active as during wakefulness. REM is also defined by phasic eye-movements and muscle atonia (paralysis). In some studies eye-movement direction during REM sleep has been shown to relate to the actual direction of gaze in the self-reported dream prior to wakening. The muscle atonia during REM prevents us from acting out our dreams; and indeed when this mechanism fails this is exactly what patients with REM-sleep behavior disorder (RBD) do.

The distribution of average time spent in specific sleep stages throughout the night is as follows:

  • Wakefulness (5%)
  • Stage 1 (2-5%)
  • Stage 2 (45-55%)
  • Stage 3 (3-8%) [SWS]
  • Stage 4 (10-15%) [SWS]
  • REM sleep (20-25%)

Shift-work and Sleep

Shift-work can be defined as work that tends to occur outside the traditional working day, usually in the hours of 7pm to 6am. Shift-work schedules can take place exclusively in the usual sleep period, occur as early morning shifts (e.g. starting at say 4-6am), or be variable (rotating) on a day-to-day or weekly basis. All three patterns may lead to sleep disturbances and impaired daytime functioning due to:

(1) Attempting to initiate sleep at a time that is inconsistent with our internal biological clock (e.g. during daylight hours)

(2) Inconsistent bed and rise times due to changing, variable shifts, including weekday versus weekend patterns.

Our biological clock has developed throughout evolution to accommodate sleep at night (when light levels are low) and maintain wakefulness and alertness during the main daylight period. Often those who work night-shifts will have difficulties sleeping during the day, after a shift, and will commonly have reduced total sleep time (sometimes by up to four hours!) and poorer sleep quality. Such workers may also feel intense sleepiness during night-shifts, due to our internal clock sending out a reduced alertness signal (after all, this is when most people are asleep). In extreme cases this excessive sleepiness may lead to occupational injury or accident.

Whilst some workers adjust well to shift work, as a group those who work shifts tend to experience greater levels of sleep disturbance, fatigue, work-related accidents, cardiovascular, gastrointestinal problems and, in women, breast cancer. Disruption of the natural circadian rhythm due to light exposure at night, and resultant impairments to sleep, are thought to contribute to the higher prevalence of ill-health in shift-workers. Environmental factors like noise and social obligations may also make sleep during the daytime very difficult.

Treatment of sleep disturbances due to shift-work focus on optimizing the adaptation to the new shift pattern so that alertness is maintained during the scheduled shift and that sleep can take place during the day. For example, work has shown that exposure to bright light prior to or during the night-shift period can help one's internal clock adapt and 're-set', permitting sleep to happen in a block later in the day. Avoiding light during the early morning (e.g. on the way home from a night-shift) may also help with this. Other recommendations may be to schedule planned naps prior to starting a night-shift, to alleviate any sleep debt, as well as to use stimulants (such as caffeine) to help maintain alertness during the shift.

Are people good at estimating how much they sleep?

It depends on how well you sleep! At the poorest end of the spectrum, research literature tells us that people with long term poor sleep tend to overestimate how long it takes them to fall asleep ('Sleep Onset Latency' or SOL), how long they are awake during the night ('Wake Time After Sleep' or WASO), and underestimate their total amount of sleep ('Total Sleep Time' or TST). This has been taken by some to mean that people with long term poor sleep “exaggerate” their problem. Little wonder that many people with sleep problems feel that their complaints are not taken seriously!

However, this discrepancy should not surprise us. People who are normally good sleepers are likely to make very similar 'errors' in estimation on those occasional nights when they sleep poorly. This suggests to me that it is not so much the person with sleep problems who is in some way at fault, rather that the task is actually quite a hard one, and one that good sleepers seldom have to perform. During the night, in the absence of stimulation and activity, time can appear to pass rather slowly (don't you know it!).

Can you think you're awake when actually you're asleep?

Yes you can, and this is a very interesting area of research – Both good and poor sleepers can have nights where they are not sure if they have been sleeping or instead spent a lot of time awake. This can be the case for a variety of reasons such as which stage of sleep you are in, the number of awakenings you have and during which stage of sleep, as well as lifestyle factors, like stress and alcohol.

There can be a gray area between wakefulness and sleep, where you have thought content but may well be asleep. Getting sleep into a block and positioned at a time that suits your internal biological clock may help with this.

There are important scientific findings which suggest that polysomnography when scored in the conventional way into sleep stages, may not identify subtle electroencephalography (EEG) characteristics that form part of the underlying pattern of poor sleep.

For example, a tendency towards waking up very, very briefly or the presence of fast EEG waves (like those we have in wakefulness or light sleep) may correspond better to the subjective experience that you are actually lying there awake.

Deep sleep

We actually have the deepest part of our sleep during the first third of the night, and there is a more rapid transition into deep sleep during this period. This is the phase of sleep where we are the least likely to wake-up from, in other words, our 'arousal threshold' is at its highest. Non-REM Stage 3 and Stage 4 together make up deep sleep, sometimes called slow-wave sleep, because electroencephalography (EEG) reveals higher amplitude waves occurring at much lower frequencies. Deep sleep is a form of synchronized sleep because the brain's electrical activity settles to a harmonized rhythm.

Deep sleep is known to be related to the amount of prior time spent awake (sleep pressure). For example, if one naps during the late afternoon for a prolonged period, the time spent in slow-wave sleep (SWS) during the subsequent night is reduced. Similarly, if one is sleep-deprived for 36 hours, then during subsequent sleep the amount of time spent in SWS is increased (as reward for the accumulated sleep debt). Sleep is therefore 'homeostatic'. In addition to minutes of time spent in deep-sleep, it is also possible to measure the intensity of deep sleep through looking at the power of slow-waves, measured as slow-wave activity. Both slow-wave sleep minutes and slow-wave activity have been found to decrease markedly with age. On average, females have more SWS than men, though there are great individual differences in the duration of SWS.

Slow-wave sleep has most often been associated with memory consolidation, next-day learning ability and vigilance. Emerging work also suggests that SWS suppression in healthy adults leads to reduced insulin sensitivity, highlighting a potential link between SWS disturbance and increased risk of diabetes. Sleep and psychiatric disorders, including insomnia, have been associated with reductions in SWS. Night-time stress, pre-sleep, has also been found to be associated with reduced SWS once asleep. Indeed, the expression of SWS is associated with low levels of the stress hormone cortisol, as well as reductions in sympathetic nervous system activity and increased parasympathetic nervous system activity. The emerging picture from experimental research is that SWS is involved in critical aspects of cognition and daytime functioning, and that it assists in the optimum maintenance of many brain and bodily functions.

Do you really get people who are 'owls' and others who are 'larks'?

You will have heard the expression 'night owl or morning lark': 'night owls' being the kind of people who come to life late in the evening and into the small hours, often having energy and feeling alert at times when most of us are beginning to feel sleepy. By way of contrast, the 'lark' is someone who is at his or her best in the morning, preferring to be up early and to make the most of the early part of the day.

These 'chronotypes' as they are sometimes called, do exist. It turns out that this preference for sleep and waking has a genetic contribution, being an expression of our internally-driven circadian rhythms (or body clock). People who have one or other of these tendencies simply have a stable 'phase position' that is slightly different from the average. Usually people adapt to their body clock tendency, and often quite like it; they make it work for rather than against themselves in their choice of occupation for example.

However, in truth, most of us are neither extreme owls (night person) or larks (morning person), tending to be somewhere in the middle. Our body clock regulates many physiological processes, like hormone secretion and body temperature, as well as sleep timing. These rhythms, that are in part driven by the light/dark cycle, determine when we are alert and sleepy, as well as the optimal time for sleep.

Sometimes the process driving the need for sleep (sleep pressure built up over the course of the day) and the body clock which, in crude terms, controls the timing of sleep, may not be working together in harmony, leading to sleep disruption. Behavioral therapies may help 'jump-start' these two processes so they work together again in optimizing sleep quality and timing.

Hormones and sleep: A two-way street

A hormone is a chemical released by a cell or gland in one part of the body that subsequently affects cells in another part of the body. In essence, hormones are chemical messengers, traveling in your bloodstream to tissues or organs. They are involved in many different bodily processes, including metabolism, growth and development, reproduction, sexual function and mood.

There is a close link between sleep and hormones. A clear demonstration of this is when women become pregnant. Pregnancy is associated with alterations to reproductive hormones, estrogen and progesterone, which typically rise throughout pregnancy and peak at term. This increase may initially be associated with elevated sleepiness in the mother, often resulting in an increase in total sleep time and daytime napping . The distribution of sleep stages, deep sleep and REM sleep, may also be altered at this time. Developing physical changes in the latter stages of pregnancy (third trimester) have also been proposed to disturb sleep in the majority of women. Similarly, during the menopause, sleep disturbance and insomnia symptoms are very common, and have been linked to decreased levels of estrogen and associated hot flashes.

Specific sleep stages may also be related to certain hormonal release. For example, it has been well documented that during deep sleep there is an increase in the release of human growth hormone, which stimulates cell reproduction and regeneration. Interestingly, a recent study by Lampl & Johnson (2011) found that infant growth spurts were associated with increased and more consolidated sleep, and the mechanism thought to explain this relates to increased deep (slow-wave) sleep and the associated increase in growth hormone.

Finally, sleep loss and sleep disturbance have also been shown to negatively impact on hormonal balance. For example, the appetite-suppressing hormone, leptin, has been found to be decreased after several nights of sleep restriction. Similarly, the appetite-stimulating hormone, ghrelin, has been found to be increased after sleep restriction. Alterations to these two hormones due to sleep loss may, therefore, encourage people to seek out extra calories!

Lampl, M., Johnson, M.L. (2011). Infant growth in length follows prolonged sleep and increased naps. SLEEP, 34(5), 641-650.

How do you measure sleep?

This is usually done in a sleep laboratory or a sleep center. Technicians attach electrodes to the head to take three types of measurement.

First, electrical activity in the brain is measured by electroencephalography (EEG). This measure is used because the EEG signals associated with being awake are different from those found during sleep. Also, the different stages of sleep can be measured using EEG.

Second, muscle activity is measured using electromyography (EMG), because muscle tone also differs between wakefulness and sleep. Once again, there are EMG differences within sleep, depending upon the stage of sleep.

Third, eye movements during sleep are measured using electro-oculography (EOG). This is a very specific measurement that helps to identify Rapid Eye Movement or REM sleep, during which we often dream. The eyeballs make characteristic movements that show us when someone is in this type of sleep.

This whole system of assessment is usually called polysomnography (PSG). The prefix 'poly' simply refers to the fact that more than one type of physiological activity is being measured.

How does the 'body clock' work?

The hormone melatonin is largely responsible for the regulation of the body clock throughout our lives. Melatonin is produced in the brain, in the pineal gland. Its production rate is dictated by natural light, so that during hours of darkness (the normal sleep period) melatonin production increases, and as morning approaches and with the coming of daylight, melatonin production is once again shut down.

Of course there is some natural variation in circadian alertness during the daytime. For example, you will probably be aware of the afternoon dip when we tend to feel temporarily rather more tired. Indeed, in some societies it is normal to have a “siesta” at this time because it also coincides with the hottest part of the day. In terms of our circadian tendencies there is much to be said for that lifestyle!

Read up on What controls our sleep pattern?

Sleep's connection to mental and physical well being

Sleep disturbances can occur on their own, but in the vast majority of people poor sleep co-exists alongside mental and/or physical health problems. The traditional view was that poor sleep was simply secondary to these other issues and that by improving the so-called 'primary' illness, poor sleep would immediately resolve. Important research conducted in the last couple of decades has revealed a much more complex picture.

It is now known, for example, that poor sleep on its own can be a risk factor for developing future illness, as well as exacerbating existing health conditions. Experimental studies with healthy good sleepers, where total sleep time is restricted, indicates that lack of sleep (or certain stages of sleep), negatively impacts next-day emotional processing and mood, pain thresholds, immune functioning and glucose metabolism. Thus both 'mind and body' are affected by alterations to sleep quality.

Such work has led to the view that sleep disturbance may help to initiate, maintain or exacerbate additional health conditions, such as psychiatric/psychological disorders, chronic pain disorders, obesity and diabetes. One somewhat “roundabout” way to assess the impact of sleep loss on co-occurring physical and mental health problems is to treat the sleep disturbance with evidence-based treatments (like cognitive behavioral therapy) and then assess what impact sleep improvement has on the additional illness. Promising work has began to emerge showing that improving sleep can also have positive effects on both depression and pain. Of course, there are clearly health-conditions whereby sleep disturbance is almost characteristic (like in some pain disorders), and improving sleep can be very challenging. However, the preliminary evidence does indicate that even in these conditions, sleep quality can be improved, and this can be achieved through addressing sleep-related behaviors and thoughts, and helping to minimize the impact of sleep disturbance on day-to-day living.

It is also worth remembering that sleep research as an applied discipline is still in its infancy, and that as we continue to understand the functions of sleep, greater understanding of and ways of improving sleep disturbance will be achieved.

If you suffer from sleep deprivation, can it cause other sleep disorders?

“Can sleep deprivation cause sleep disorders?”

Many symptoms of sleep disorders have been found to be triggered by a lack of sleep. For example, sleep-walking events, sleep paralysis (the inability to initiate muscle movements at the onset of sleep or when waking-up) and sleep-related hallucinations (experienced when initiating sleep or waking-up from sleep) can often occur after several nights of poor sleep or irregular sleep-wake schedules.

Perhaps somewhat paradoxically, sleep loss may also exacerbate or be involved in the development of insomnia. For example, when someone has a very poor night of sleep, they may as a consequence feel more stressed or anxious the next day. These feelings often concern their ability to function, particularly whether or not they will obtain sleep the subsequent night. If, when approaching bedtime, these thoughts become increasingly salient, they may increase physiological and mental arousal, ultimately impairing the ability to initiate and maintain sleep. Thus, it is clear that sleep and how we feel and function during the day are highly interlinked.

Is there a link between 'dream time' and sleep length?

There is no clear relationship between length of sleep per se and 'dream-time'; it would depend on a number of factors, including (and especially) how long you would spend in a particular stage of sleep (REM sleep, for example, where we most often experience dream content).

However, even spending longer amounts of time in REM sleep would not necessarily mean that your dream would seem 'longer'. Because of the bizarreness of our dream content, and from what we know of brain activity during REM sleep, the processes that keep track of time and the sequential ordering of events that take place during waking experience are unlikely to be similar.

One exception to this however, might be lucid dreaming – a rare ability where the dreamer can control dreamed actions at will.

Menopause and sleep problems

The menopausal transition is characterized by considerable hormonal changes, most notably, reductions in the level of the hormone 'oestradiol' and increases in follicle stimulating hormones (FSH). During this period the majority of women experience symptoms such as hot flashes and night-sweats as well as changes in mood. Reports of difficulty falling asleep and staying asleep tend to increase during the menopause transition; and hot-flash frequency is associated with reports of poor sleep quality.

A recent study (Campbell et al., 2011) found that late peri-menopausal women (women in the transitional period just before entering the menopause) and post menopausal women, compared with early menopausal women and pre-menopausal women, showed increased EEG beta power during sleep (both REM and NREM sleep).

'Beta power' during sleep can be interpreted as evidence of enhanced arousal, potentially leading to the subjective impression of being awake when, by conventional scoring methods, we are considered categorically asleep. In addition, subjective reports of sleep quality were also found to be reduced over the menopausal transition.

Ongoing work is beginning to assess what treatments may be effective for sleep disturbance in menopausal women; with recent controlled studies looking at the effects of hormone replacement therapy (HRT), anti-depressants, hypnotic sleeping pills, valerian, and non-pharmacological interventions, like yoga, on measures of sleep quality.

Campbell, I.G., Bromberger, J.T., Buysse, D.J., Hall, M.H., Hardin, K.A., Kravitz, H.M., Matthews, K.A., Rasor, M.O., Utts, J., Gold, E. (2011). Evaluation of the association of menopausal status with delta and beta activity during sleep. SLEEP, 34(11), 1561-1568.

Seasonal Effects on Sleep

Seasonal variation can have an impact on our sleep/wake cycle. For example, studies have shown that during summer sleep times, core body temperature and melatonin secretion (a hormone involved in the timing of sleep) all tend to be slightly advanced. This means that they tend to occur earlier in the night relative to winter months (e.g. when studied under controlled experimental conditions, people tend to go to bed earlier and wake-up earlier in summer). The main reason for this is due to the photo-period (length of light exposure), which tends to be greater in the summer months. Light exposure early in the morning can impact our internal biological clock, shifting the timing of the sleep window.

A recently published study by Friborg et al. (2011) supports the idea of seasonal variation in sleep due to daylight duration. The study compared Norway, a country that undergoes large seasonal variation, with Ghana, a country that undergoes very little seasonal variation (due to its position close to the equator). Seasonal effects were found for Norway, with bed and rising times being earlier in summer, while insomnia, fatigue, and low mood were more prevalent in winter. These winter-summer seasonal differences were not found to be present for Ghana.

Another related and well-studied phenomenon is Seasonal Affective Disorder (SAD), this is where people experience a change in mood due to changing seasons, but who otherwise are typically in good mental health for the rest of the year. In line with the study mentioned above, the most common finding is that depressive symptoms tend to peak in winter. For patients with SAD, there is evidence that prescribed light exposure (through a so-called lightbox) can reduce depressive symptoms. The timing of light exposure is very important and should be guided by one's doctor or health professional.

Friborg, O., Bjorvatn, B., Amponsah, B., Pallesen, S. (2012). Associations between seasonal variations in day length (photoperiod), sleep timing, sleep quality and mood: a comparison between Ghana (5°) and Norway (69°). Journal of Sleep Research, 21(2), 176-184.

What controls our sleep pattern?

Sleep is an automatic process and therefore out of our own direct, voluntary control. Whether awake or asleep we are at the mercy of two biological processes: Sleep Homeostasis, commonly known as (1) 'Sleep Pressure'; and (2) The Circadian Rhythm, otherwise known as the 'Body Clock'. These two processes work in harmony to promote good consolidated sleep at night.

Sleep pressure can be thought of as the brains pressure and need for sleep, becoming greater with increasing amounts of time spent awake. In this way, the pressure to sleep is directly related to the amount of time that we have been awake. For example, when we wake-up in the morning after a good night's sleep, we will have a very low sleep pressure or 'need to sleep'. As we continue throughout the day, sleep pressure will begin to accumulate (a bit like an hourglass egg-timer). At the end of a full day, at bedtime, we will have a great amount of pressure to sleep. By going to bed and having another good night's sleep, then sleep pressure will be reset for the start of the next day.

The Circadian Rhythm (Body clock) is an internally generated biological rhythm that allows a number of processes to rise and fall over the twenty four hour period. Commonly, its effects are mostly realized with jet lag when one travels through many different time zones rapidly. This is when the circadian rhythm is out of synchrony with the new environment and can take a number of days to go back to normal.

In a good sleeper, who is in-synch with the environment, the circadian rhythm will naturally rise in the early morning, promoting wakefulness and alertness – this is sometimes known as the alerting force. As the day continues, the circadian rhythm will promote wakefulness until it reaches a peak at about midday. After this time, the circadian rhythm will start to dip. This initial fall is known as the 'post lunch dip' (you may be familiar with greater feelings of sleepiness after lunch) and as a time for a siesta in other cultures. As we continue through the day, the circadian rhythm continues to fall and does not promote as much arousal as before. With the onset of bedtime and sleep, the circadian rhythm drops to the lowest level and helps to maintain sleep. In this way, sleep pressure is very high, while the alerting effect of our body clock is low, creating the optimal opportunity to sleep.

After this low point, the circadian rhythm will then rise again in anticipation for the next day. The body clock is difficult to manipulate and may be disrupted in a number of poor sleep conditions.

What is REM sleep?

In 1953 two researchers in Chicago, Dr. Nathaniel Kleitman and his young assistant Dr. Aserinsky, made a crucial discovery about sleep. They noticed that there was a form of sleep during which the eyeballs move rapidly, whereas the rest of the body was pretty much paralyzed. Intriguingly, the electrical brain activity of this period of sleep was also observed to be very similar to that seen during wakefulness. Two names for this stage of sleep subsequently emerged – Rapid eye movement sleep (or REM sleep) to referring to the eye movements, and Paradoxical sleep, referring to the paradoxically enhanced brain activity being very similar to that in the waking brain.

Research emerged showing that REM sleep was related to dream content: people woken up during this phase of sleep are more likely to report dream fragments relative to every other stage of sleep. The content of dreams during REM sleep also tend to be more bizarre, vivid and emotionally charged. Brain imaging studies have since provided further insight into the physiological basis of dreams. Parts of the brain involved in the processing of emotional information are very active, as are areas known to be involved in visual imagery. While these parts of the brain are intensely active, other parts of the brain, typically those involved in self-awareness and executing control, actually show reduced activity. This may help us understand why dreams often have very little logical order and that we are usually unable to control what happens during our dreams. Finally, the eye movements that occur during REM sleep have also been shown to closely follow the dreamed action. For example, if a person is climbing a ladder in their dream, often the recorded eye movements will follow the dreamed movement (i.e. change in gaze upwards as the individual begins to climb up several rungs).

The muscle paralysis (or muscle atonia) that takes place during REM sleep prevents us from acting-out our dreams. A neurological condition called REM-sleep Behavior Disorder (or RBD) is characterized by a failure of this muscle atonia (or muscle paralysis), leading patients to literally act out their dreams. Acting-out dreams may lead to self-injury or injury to a proximal bed-partner. When woken from the dream, patients with RBD will tend to have good recall of the recent dream content (this is in contrast to those who sleep-walk, where there is usually poorer recall of thought content prior to awakening). Studies have discovered a link showing that a sizeable number of those with RBD may go on to develop Parkinson's disease or dementia in the future.

What is melatonin?

Melatonin is a naturally occurring hormone which the brain produces in the late evening and throughout the night. It is associated with the dark period of the light-dark cycle. Some studies suggest that melatonin in tablet form, several hours before bedtime, can help people get to sleep more quickly. However, again there are few long-term studies, so melatonin does not offer a solution for persistent poor sleep. Some people prefer the idea of a 'natural' product over a sleeping pill, but like sleeping pills, the benefits of melatonin seem to be lost almost immediately when people stop taking medication.

Light at night may inhibit the production of melatonin and lead to a delay in sleep onset and possibly sleep fragmentation. There is a growing concern that adolescents (and adults) are being exposed to light from phones and computers, for example, when in bed late at night, impacting on melatonin and sleep-onset.

As ever please consult your doctor before considering any change in your medication regime.


'Normal' (or primary) snoring, not associated with episodes of apnoea or hypoventilation, is a respiratory sound generated in the upper airway during sleep, particularly deep (slow-wave) sleep and REM sleep. Nearly everyone will snore at some point in their life, and habitual snoring is also very common, affecting up to 40% of adult men and 25% of women. It is linked to obesity as well as nasal obstruction. Decreased muscle tone during sleep can result in constriction and vibration of tissues in the upper-airway, particularly those of the uvula and soft palate.

On its own, snoring typically does not cause excessive daytime sleepiness or insomnia, but it may affect one's bed partner, and therefore can often require medical treatment. Available interventions include nasal air strips (to keep the nostrils open during sleep), dental splints (to move the jaw and tongue forward), and in extreme cases palatal surgery (e.g. reducing the amount of soft palate and/or removing the tonsils to help minimize collapsibility of the oropharyngeal segment).

Why do we need to sleep?

Sleep is not an optional extra in life; it is a fundamental requirement. In fact, you could survive for three times as long without food as you could without sleep. Much of what we know about the importance of sleep comes from experiences of people who have taken part in sleep-deprivation experiments. That is, where insufficient sleep, or no sleep, has been taken over successive 24-hour periods. The bottom line is that when people are sleep deprived they are not able to function properly during the day. So, one simple answer to the question “What is sleep for?” would be that the purpose of sleep is to make sure of good-quality daytime functioning.

Sleep has its physical, mental and emotional processing components, so when we have had no sleep, or insufficient sleep, these processes are not able to do their work so effectively. In physical terms we will feel lethargic and sleepy, mentally we become slowed down with poorer concentration and memory, and emotionally we may become irritable and rather down, though sometimes with excitable bursts of hyperactivity too.

Stress and sleep

Experimental studies that manipulate or induce stress in humans, using both physical and psychological stimuli, have shown that stress negatively interferes with the ability to initiate and maintain sleep. It also affects the composition of sleep stages, reducing deep sleep, leaving us in lighter phases of sleep and more vulnerable to environmental disruption, such as light and noise.

Traumatic life events may also impact the content of our dreams, leading to chronic, sometimes nightly nightmares, where the event is replayed over and over again – this can be very common in those diagnosed with post-traumatic stress disorder (PTSD).

It is also often the case that those with chronic sleep disturbance (insomnia) tend to anchor the onset of their sleep problem to a negative stressful life event, typically around family, health or the work-place. Individuals respond to stress in different ways, and this is likely to have a genetic contribution. For those with a vulnerability to stress-related sleep disturbance, it is likely that a stressful event results in excessive physiological arousal (e.g. increased heart rate, stress hormones) and/or psychological arousal (e.g. anxiety), which will directly impact regions of the brain responsible for initiating sleep and inhibiting wakefulness.

While the majority of us will experience a poor night of sleep from time-to-time, as consequence of daily stressors, a smaller proportion will subsequently develop chronic sleep problems. The reasons for the transition from acute to chronic sleep disturbance is a matter of intense research, but both psychological and physiological factors (and their interaction) are likely to be involved. We know, for example, that cognitive behavioral techniques, addressing sleep-related thoughts and behaviors, can be very effective in improving sleep in those with poor sleep. Similarly, sleeping pills which affect neurochemicals involved in the regulation of sleep, are also effective in the short-term improvement of broken sleep.

What accounts for unrefreshing sleep?

Often we think of reductions in total sleep time and fragmented sleep as the major contributors to feeling unrefreshed the next day. While this is true (and has been shown extensively in controlled studies) it is possible to have normal total sleep time but for alterations to the composition and distribution of certain stages of sleep to affect how we feel and function. For example, a recent study found that 'knocking-out' deep- or slow-wave sleep (by using sound to shift the brain into lighter phases of sleep) impairs the ability to form new memories the next day and leads to increased lapses during attention tasks. It has also been shown that micro-changes in the scalp-recorded Electroencephalography (EEG) (the main measure of objective sleep) can be associated with poor sleep quality, without there necessarily being any clear reductions in total sleep time

So in short, the quality as well as quantity of our sleep is what matters – and it's why a reliable schedule is a critical part of being a healthy sleeper.

See this article for more information on getting a good night's sleep

Sleep deprivation symptoms

Research tells us that sleep deprived people find it more difficult to perform to their full potential during the day. You may well be familiar with the feeling of gritty eyes in the morning, but there are many other symptoms of a lack of sleep, which you may not be aware of.

Humans have a natural sleep drive which builds throughout the day whilst we remain awake. This grows into the evening until we eventually feel an overwhelming 'pressure' to sleep. Sleep pressure is then relieved as we sleep, until we wake up and the process starts once again.

Insufficient sleep, however, has been found to exacerbate the symptoms of several sleep disorders. Sleep paralysis and sleepwalking, for example, are more likely to occur after several nights of poor sleep or irregular sleep-wake schedules.

Symptoms of sleep deprivation

Lack of sleep symptoms tend to vary according to how long and how often we are sleep deprived within a certain period of time. In general though, it is fair to say that sleep deprivation affects us on physical, mental and emotional levels.

Experimental studies with healthy good sleepers, where total sleep time is restricted, indicate that lack of sleep (or certain stages of sleep), negatively impacts a huge range of functions, including emotional processing, pain thresholds, immune functioning and glucose metabolism.

Physical symptoms

Physically, lack of sleep may leave us struggling with low energy levels during the day. Findings from the Great British Sleep Survey show that poor sleepers are twice as likely to feel fatigued. In fact, 88% of poor sleepers struggled with reduced energy compared to only 29% of 'good' sleepers.

Sleepiness, as opposed to 'tiredness', is another physical symptom, demonstrated by our propensity to fall asleep or trouble to stay awake during the day.

Although not terribly common, lightheadedness can also be a consequence of sleep deprivation. Often, headaches and tension are found to increase after poor sleep; occasionally this may be accompanied by feelings of dizziness and light-headedness. It is recommended that you consult your doctor if you experience symptoms such as this.

Mental symptoms

Mentally, poor sleep may result in poorer concentration and memory. In research studies, sleep deprived people have shown impairments in both sustained attention and memory performance.

Of those who participated in our survey of the nation's sleep, poor sleepers were 62% more likely to report struggling to concentrate or 'think clearly'.

Emotional symptoms

Emotionally, we may find ourselves more irritable and lower in mood, as a result of poor or insufficient sleep. Research has consistently found that sleep deprived people show less stable patterns of behavior and are more likely to be emotionally labile. Indeed, the Great British Sleep Survey revealed those suffering from insufficient sleep were twice as likely to suffer from low mood as those who sleep well.

Whilst only restorative sleep can relieve signs of sleep deprivation, some people may have trouble inducing or maintaining sleep. Given how much our quality of sleep affects us the next day, it is important to seek treatment for any sleep problem that you are experiencing.

Cognitive behavioral therapy (CBT) has been shown to be highly effective in helping people establish a regular sleep schedule. Further to this, the clinical trial of the Sleepio program found participants' mood, alongside their sleep, to have improved at two months post-treatment.

Participants also saw their energy and daytime well-being more than double compared to levels prior to embarking on the Sleepio course.

Durmer, J.S., Dinges, D.F. (2005). Neurocognitive consequences of sleep deprivation. Seminars in Neurology, 25(1), 117-129.

How long can you go without sleep?

What's the longest you have gone without sleep?

Like breathing, sleep is a fundamental human requirement. It has even been said that one could survive for three times as long without food as one could without sleep.

Despite research such as this, there is still much which remains unexplained around the importance of sleep. In fact, in the study described above, it cannot be established that sleep deprivation was the cause of these animals' deaths. A number of the methods used in research can be identified as potential causes – the animals being wakened using an electric shock each time they lapsed into sleep, for example.

The question of how long a human can go without sleep remains unanswered by research. We are aware however, of cases outside scientific study where people have died after periods of no sleep at all.

Fatal familial insomnia

Fatal familial insomnia (FFI) is a rare, and ultimately terminal, genetically inherited prion disease. Once an individual begins to show the symptoms of FFI, starting with insomnia, the illness progresses quickly and further symptoms emerge. These symptoms include hallucinations, weight loss and finally dementia before their death.

The best-known case of FFI is that of Michael Corke, who died after 6 months of total sleep deprivation. As with the clinical experiments on animals, it is very difficult to determine whether lack of sleep is the definitive cause of death in people suffering from FFI. Thus, we cannot conclude that 6 months really is how long you can go without sleep before you die.

So, how long can you survive without sleep?

Ultimately, we do not know. Sleep science is a young discipline and only in the last few decades have we really started to make advances in our understanding of the importance and functions of sleep. In the 1960s a high school student named Randy Gardner set out to break the world record for the longest time spent awake. During the experiment he contracted problems with eyesight as well as various cognitive deficiencies, such as speech and memory problems (Ross, 1965). Towards the end of the experiment he also started to hallucinate. These symptoms emerged within just 11 days.

What we do know is that it is unwise to ignore our need for sleep. The negative side effects of partial sleep deprivation have been observed in in countless research studies and it is safe to assume that these would only be worsened by prolonged total sleep deprivation.


Everson, C.A., Bergmann, B.M., Rechtschaffen, A. (1989). Sleep deprivation in the rat: III. total sleep deprivation. Sleep, 12(1), 13-21.

Ross, J.J. (1965). Neurological findings after prolonged sleep deprivation. Archives of Neurology, 12(4), 399-403.

Lack of sleep effects

It is perfectly normal to worry about the effects of a lack of sleep. In fact, the findings of the Great British Sleep Survey tell us that 68% of poor sleepers surveyed were bothered by thoughts about how they would cope the next day.

It stands to reason that these worries may not be completely unfounded and years of research on good sleepers has shown that, although the effects of sleep deprivation vary by person, we are all affected following a night of insufficient or poor sleep. Just some of the areas in which you may be affected are mood and emotional processing, pain thresholds, functioning of the immune system and glucose metabolism.

Lack of sleep and sport

Research into how lack of sleep affects sports performance, further emphasizes the potential impact of sleep deprivation. In particular, impairments to glucose metabolism may prove problematic for athletes who rely on their energy supplies to perform to the best of their ability. In fact, athletes may benefit from even more sleep than the average person; a recent study found that collegiate basketball players saw their performance improve significantly after sleeping for at least 10 hours (Mah et al. 2011).

Lack of sleep and health

Effects of sleep deprivation on the skin should not be overlooked either. A lack of sleep can impair the body's ability to fend off diseases (Irwin et al. 1994) and inflammation. One recent study on rats showed that partial sleep deprivation led to worsening of psoriasis-related biomarkers thus possibly increasing the risk of the subject developing psoriasis (Hirotsu et al., 2012).

Finally, a chronic lack of sleep has also been linked to weight gain. A study by Spiegel et al. (2004) found that restricting sleep in 12 healthy men for two days, from 10 to 4 hours, resulted in a reduction in leptin (a hormone involved in feeling 'full' after eating) and elevations in ghrelin (hormone involved in stimulating feelings of hunger). These hormonal changes were also accompanied by self-reported increases in appetite and hunger, particularly for high calorie foods.

Alongside this, simply being awake longer and at odd times may, for example, give us more opportunity to eat and limit our ability and motivation to exercise!

How bad are the effects of a lack of sleep?

However, the good news is that the effects of a lack of sleep may not be as bad as one would expect. How refreshed you feel in the morning will depend both on the continuity and architecture of sleep. We know that the first few hours of sleep are the most beneficial, in terms of physical restoration, which is why one will sometimes wake up after 3 hours of sleep and feel well rested. It is the exclusion of certain sleep stages that was linked to many of the negative effects of sleep deprivation discussed.

Fortunately, we do not need to repay sleep loss on an hour-for-hour basis. The best evidence we have from experimental studies of sleep deprivation suggests that we need to make up less than one-third of our lost hours. Furthermore, the sleep we get on recovery nights may be deeper and more restorative.


Mah, C.D., Mah, K.E., Kezirian, E.J., Dement, W.C. (2011). The effects of sleep extension on the athletic performance of collegiate basketball players. Sleep, 34(7), 943-950.

Irwin, M., Mascovich, A., Gillin, J.C., Willoughby, R., Pike, J. & Smith, T.L. (1994). Partial sleep deprivation reduces natural killer cell activity in humans. Psychosomatic Medicine, 56(6), 493-498.

Hirotsu. C., Rydlewski, M., Araujo, M.S., Tufik, S., Andersen, M.L. (2012). Sleep loss and cytokines levels in an experimental model of psoriasis. PloS ONE, 7(11): e51183.

Spiegel, K., Tasali, E., Penev, P., Van Cauter, E. (2004). Brief communication: sleep curtailment in healthy young men is associated with decreased leptin levels, elevated ghrelin levels and increased hunger and appetite. Annals of Internal Medicine, 141(11), 846-850.

What happens if a person does not sleep?

Firstly, you will have missed out on one of the biggest benefits of sleep – feeling fresh in the morning!

Secondly, sleep is vital for healthy physical, mental and emotional processing. When we go without sleep, or have insufficient sleep, our bodies struggle to perform to their full potential and, as a consequence, we can expect impairments to our next-day physical and mental performance.

What happens to the body if you don't get enough sleep?

Due to a close link between certain hormones and sleep, not sleeping has the potential to cause imbalances in hormone activity. Human Growth Hormone, for example, peaks during sleep meaning that insufficient sleep may affect growth and cell-repair throughout the body.

In addition to growth, your metabolism may be affected as well. Studies in which healthy individuals have been sleep restricted have shown that there are alterations to hormones involved in the regulation of appetite and an accompanying increase in seeking out food, as well as glucose metabolism.

What happens in the brain when you don't sleep?

Overall, research has suggested that normal functioning is likely to be hindered by loss of sleep. Repercussions such as reduced energy levels with bursts of euphoria, unstable moods and excessive sleepiness during the day have all been observed in people who haven't slept.

Excessive sleepiness can be especially hindering and even dangerous as it tends to be preceded by frequent lapses in focus before individuals fall into a short episodes of sleep, also known as 'microsleeps'. These episodes are a known contributing factor to traffic accidents with drowsy drivers falling asleep at the wheel (Boyle et al. 2008).

Whilst we can recover from not sleeping very quickly, it can have negative long-term consequences for our health. Chronic poor and restricted sleep are known, for example, to be associated with the development of illness, notably cardiovascular disease, diabetes, hypertension and certain types of cancers.

Research on sleep deprivation

The most well known experiment on total sleep deprivation involved a teenager called Randy Gardner, who managed to maintain wakefulness for 11 days. During this period, he experienced problems with his working memory, speech and eventually hallucinations.

It is safe to say that keeping yourself awake long after feeling the pressure to sleep is unwise. Sleeping is not something humans can whether or not to do – it is essential for facilitating normal functioning.


Boyle, L.N., Tippin, J., Paul, A., Rizzo, M. (2008). Driver performance in the moments surrounding a microsleep. Transportation Research Part F: Traffic Psychology and Behaviour, 11(2), 126-136.

The science of sleep

Sleep science is still a relatively new area of research and to this day, scientists struggle both to define sleep, and to agree on its core function (Kirsch 2011).

Great strides have been made towards understanding the processes involved in sleep over the last decade however, and advancing technologies continue to allow us to monitor and study sleep in depth.

The history of sleep research

Modern research into sleep is said to have begun with an American scientist, Dr. Nathaniel Kleitman.

Dr. Kleitman was the first person to focus research solely on the science of sleep, founding the first ever sleep laboratory at the University of Chicago, and publishing his findings for those interested in the field to learn from.

Progress in sleep research was later made by Dr. William Charles Dement, one of the first scientists to measure brain activity during sleep using electroencephalography, or 'EEG'. This allowed the stages of sleep, or 'sleep architecture' to be observed, and subsequently defined, for the first time.

What is sleep?

Carskadon and Dement (2011) define sleep as a “reversible behavioral state of perceptual disengagement from, and unresponsiveness to, the environment”.

It is possible to divide sleep into two, broadly defined, states: rapid-eye movement (REM) sleep and non-REM (NREM) sleep. We normally enter sleep through NREM sleep but alternate between NREM and REM sleep throughout the night. This process is known as a 'sleep cycle'.

Sleep cycles tend to last approximately 90 minutes, with good sleepers generally going through 4-5 sleep cycles on an average night. The composition of these cycles changes throughout the night, with the first third of the night tending to be made up of greater amounts of slow-wave (deep) sleep. In contrast, towards the end of the night it is REM sleep which dominates, followed by changes in body temperature.

The distribution of average time spent in specific sleep stages throughout the night is as follows:

  • Wakefulness (5%)
  • Stage 1 (2-5%)
  • Stage 2 (45-55%)
  • Stage 3 (3-8%) [slow-wave sleep]
  • Stage 4 (10-15%) [slow-wave sleep]
  • REM sleep (20-25%)

Measuring sleep

Sleep assessments, usually carried out in sleep laboratories or sleep centers, include three different types of measurement:

  • Electrical activity in the brain is measured by electroencephalography (EEG), is used to differentiate between wakefulness, sleep, and the different stages of sleep.
  • Muscle activity is measured using electromyography (EMG), because muscle tone also differs between wakefulness and sleep.
  • Eye movements during sleep are measured using electro-oculography (EOG). This is a very specific measurement that helps to identify Rapid Eye Movement or REM sleep

This system of assessment is referred to as polysomnography, or 'PSG'.


Carskadon, M.A., Dement, W.C. (2011). Monitoring and staging human sleep. In Kryger, M.H., Roth, T., Dement, W.C. (Eds.), Principles and practice of sleep medicine, 5th edition, p16-26. St. Louis: Elsevier Saunders.

Kirsch, D.B. 2011. There and back again: a current history of sleep medicine. Chest, 139(4), 939-946.