Sleep is a naturally recurring state of relatively suspended sensory and motor activity, characterized by total or partial
unconsciousness and the inactivity of nearly all
voluntary muscles.
It is distinguished from quiet
wakefulness by a decreased ability to react to
stimuli, and it is more easily reversible than
hibernation or
coma. Sleep is a heightened
anabolic state, accentuating the growth and rejuvenation of the immune, nervous, skeletal and muscular systems. It is observed in all mammals, all birds, and many reptiles, amphibians, and fish. In humans, other mammals, and a substantial majority of other animals that have been studied (such as some species of fish, birds, ants, and
fruit flies), some form of sleep may be essential for survival.
The purposes and mechanisms of sleep are only partially clear and are the subject of intense research.
Physiology
Stages of sleep
In mammals and birds, sleeping is cut into two broad types: rapid eye movement (REM) and non-rapid eye movement (NREM or non-REM) sleep. Each type has a distinct set of associated physiological, neurological, and psychological features. The American Academy of Sleep Medicine (AASM) further divides NREM into three stages: N1, N2, and N3, the last of which is also called delta, or slow-wave, sleep (SWS).
Sleep cycles through the night, with deep sleep early on and more REM (marked in red) toward morning.
Stage N3 sleep; EEG highlighted by red box. Thirty seconds of deep sleep, here with greater than 50% delta waves.
REM sleep; EEG highlighted by red box; eye movements highlighted by red line. Thirty seconds of sleep.
Sleep proceeds in cycles of REM and NREM, the order normally being N1 → N2 → N3 → N2 → REM. There is a greater amount of deep sleep (stage N3) early in the night, while the proportion of REM sleep increases later in the night and just before natural awakening.
The stages of sleep were first described in 1937 by Alfred Lee Loomis and his coworkers, who separated the different electroencephalography (EEG) features of sleep into five levels (A to E), which represented the spectrum from wakefulness to deep sleep. In 1953, REM sleep was discovered as distinct, and thus William Dement and Nathaniel Kleitman reclassified sleep into four NREM stages and REM. The staging criteria were standardized in 1968 by Allan Rechtschaffen and Anthony Kales in the “R&K sleep scoring manual.” In the R&K standard, NREM sleep was divided into four stages, with slow-wave sleep comprising stages 3 and 4. In stage 3, delta waves made up less than 50% of the total wave patterns, while they made up more than 50% in stage 4. Furthermore, REM sleep was sometimes referred to as stage 5.
In 2004, the AASM commissioned the AASM Visual Scoring Task Force to review the R&K scoring system. The review resulted in several changes, the most significant being the combination of stages 3 and 4 into Stage N3. The revised scoring was published in 2007 as The AASM Manual for the Scoring of Sleep and Associated Events. Arousals and respiratory, cardiac, and movement events were also added.
Sleep stages and other characteristics of sleep are commonly assessed by polysomnography in a specialized sleep laboratory. Measurements taken include EEG of brain waves, electrooculography (EOG) of eye movements, and electromyography (EMG) of skeletal muscle activity. In humans, each sleep cycle lasts from 90 to 110 minutes on average, and each stage may have a distinct physiological function. This can result in sleep that exhibits loss of consciousness but does not fulfill its physiological functions (i.e., one may still feel tired after apparently sufficient sleep).
NREM sleep
According to the 2007 AASM standards, NREM consists of three stages. There is relatively little dreaming in NREM.
Stage N1 refers to the transition of the brain from alpha waves having a frequency of 8 to 13 Hz (common in the awake state) to theta waves having a frequency of 4 to 7 Hz. This stage is sometimes referred to as somnolence or drowsy sleep. Sudden twitches and hypnic jerks, also known as positive myoclonus, may be associated with the onset of sleep during N1. Some people may also experience hypnagogic hallucinations during this stage, which can be troublesome to them. During N1, the subject loses some muscle tone and most conscious awareness of the external environment.
Stage N2 is characterized by sleep spindles ranging from 11 to 16 Hz (most commonly 12–14 Hz) and K-complexes. During this stage, muscular activity as measured by EMG decreases, and conscious awareness of the external environment disappears. This stage occupies 45% to 55% of total sleep in adults.
Stage N3 (deep or slow-wave sleep) is characterized by the presence of a minimum of 20% delta waves ranging from 0.5 to 2 Hz and having a peak-to-peak amplitude >75 μV. (EEG standards define delta waves to be from 0 – 4 Hz, but sleep standards in both the original R&K, as well as the new 2007 AASM guidelines have a range of 0.5 – 2 Hz.) This is the stage in which parasomnias such as night terrors, nocturnal enuresis, sleepwalking, and somniloquy occur. Many illustrations and descriptions still show a stage N3 with 20%-50% delta waves and a stage N4 with greater than 50% delta waves; these have been combined as stage N3.
REM sleep
Rapid eye movement sleep, or REM sleep, accounts for 20%–25% of total sleep time in most human adults. The criteria for REM sleep include rapid eye movements as well as a rapid low-voltage EEG. Most memorable dreaming occurs in this stage. At least in mammals, a descending muscular atonia is seen. Such paralysis may be necessary to protect organisms from self-damage through physically acting out scenes from the often-vivid dreams that occur during this stage.
Timing
The human biological clock
Sleep timing is controlled by the circadian clock, sleep-wake homeostasis, and in humans, within certain bounds, willed behavior. The circadian clock—an inner timekeeping, temperature-fluctuating, enzyme-controlling device—works in tandem with adenosine, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine is created over the course of the day; high levels of adenosine lead to sleepiness. In diurnal animals, sleepiness occurs as the circadian element causes the release of the hormone melatonin and a gradual decrease in core body temperature. The timing is affected by one’s chronotype. It is the circadian rhythm that determines the ideal timing of a correctly structured and restorative sleep episode.
Homeostatic sleep propensity (the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep. Along with corresponding messages from the circadian clock, this tells the body it needs to sleep. Sleep offset (awakening) is primarily determined by circadian rhythm. A person who regularly awakens at an early hour will generally not be able to sleep much later than their normal waking time, even if moderately sleep-deprived.
Sleep duration is affected by circadian rhythm which is regulated by the gene DEC2. Some people have a mutation of this gene; they sleep two hours less than normal. Neurology professor Ying-Hui Fu and her colleagues bred mice that carried the DEC2 mutation and slept less than normal mice.
Optimal amount in humans
Adult
The optimal amount of sleep is not a meaningful concept unless the timing of that sleep is seen in relation to an individual’s circadian rhythms. A person’s major sleep episode is relatively inefficient and inadequate when it occurs at the “wrong” time of day; one should be asleep at least six hours before the lowest body temperature. The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening:
- maximum concentration of the hormone melatonin, and
- minimum core body temperature.
Human sleep need can vary by age and among individuals, and sleep is considered to be adequate when there is no daytime sleepiness or dysfunction.
A University of California, San Diego psychiatry study of more than one million adults found that people who live the longest self-report sleeping for six to seven hours each night. Another study of sleep duration and mortality risk in women showed similar results. Other studies show that “sleeping more than 7 to 8 hours per day has been consistently associated with increased mortality,” though this study suggests the cause is probably other factors such as depression and socioeconomic status, which would correlate statistically. It has been suggested that the correlation between lower sleep hours and reduced morbidity only occurs with those who wake after less sleep naturally, rather than those who use an alarm.
Main health effects of sleep deprivation, indicating impairment of normal maintenance by sleep.
Researchers at the University of Warwick and University College London have found that lack of sleep can more than double the risk of death from cardiovascular disease, but that too much sleep can also be associated with a doubling of the risk of death, though not primarily from cardiovascular disease. Professor Francesco Cappuccio said, “Short sleep has been shown to be a risk factor for weight gain, hypertension, and Type 2 diabetes, sometimes leading to mortality; but in contrast to the short sleep-mortality association, it appears that no potential mechanisms by which long sleep could be associated with increased mortality have yet been investigated. Some candidate causes for this include depression, low socioeconomic status, and cancer-related fatigue… In terms of prevention, our findings indicate that consistently sleeping around seven hours per night is optimal for health, and a sustained reduction may predispose to ill health.”
Furthermore, sleep difficulties are closely associated with psychiatric disorders such as depression, alcoholism, and bipolar disorder. Up to 90% of adults with depression are found to have sleep difficulties. Dysregulation found on EEG includes disturbances in sleep continuity, decreased delta sleep and altered REM patterns with regard to latency, distribution across the night and density of eye movements.
Hours by age
Children need more sleep per day in order to develop and function properly: up to 18 hours for newborn babies, with a declining rate as a child ages. A newborn baby spends almost 9 hours a day in REM sleep. By the age of five or so, only slightly over two hours is spent in REM.
| Age and condition |
Average amount of sleep per day |
| Newborn |
up to 18 hours |
| 1-12 months |
14-18 hours |
| 1-3 years |
12-15 hours |
| 3-5 years |
11-13 hours |
| 5-12 years |
9-11 hours |
| Adolescents |
9-10 hours |
| Adults, including elderly |
7-8(+) hours |
| Pregnant women |
8(+) hours |
Sleep debt
Sleep debt is the effect of not getting enough rest and sleep; a large debt causes mental, emotional, and physical fatigue. It is unclear why a lack of sleep causes irritability.
Sleep debt results in diminished abilities to perform high-level cognitive functions. Neurophysiological and functional imaging studies have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure.
Scientists do not agree on how much sleep debt it is possible to accumulate; whether it is accumulated against an individual’s average sleep or some other benchmark; nor on whether the prevalence of sleep debt among adults has changed appreciably in the industrialized world in recent decades. It is likely that children are sleeping less than previously in Western societies.
Genetics
A considerable amount of sleep-related behavior is apparently hard-wired into human biology—humans in all cultures get tired, require sleep for good health, and have similar symptoms when sleep deprived. Scientific research has identified some genetic variations, including:
- A mutation that moves consolidated sleep earlier, resulting in a sleep cycle from 7:30pm to 3:30am.
- A mutation in BHLHB3 which apparently reduces the amount of sleep needed for healthy living from 8 hours to only 6.
Functions
The multiple theories proposed to explain the function of sleep reflect the as-yet incomplete understanding of the subject. It is likely that sleep evolved to fulfill some primeval function and took on multiple functions over time. (As an analogy, the larynx in all mammals controls the passage of food and air, but may have descended in humans to take on speech capabilities in addition.)
It has been pointed out that, if sleep were not essential, one would expect to find 1) animal species that do not sleep at all, 2) animals that do not need recovery sleep when they stay awake longer than usual, and 3) animals that suffer no serious consequences as a result of lack of sleep. No animals have been found to date that satisfy any of these criteria.
Some of the many proposed functions of sleep are as follows.
Restoration
Wound healing has been shown to be affected by sleep. A study conducted by Gumustekin et al. in 2004 shows sleep deprivation hindering the healing of burns on rats.
It has been shown that sleep deprivation affects the immune system. In a study by Zager et al. in 2007, Â rats were deprived of sleep for 24 hours. When compared with a control group, the sleep-deprived rats’ blood tests indicated a 20% decrease in white blood cell count, a significant change in the immune system. It is now possible to state that “sleep loss impairs immune function and immune challenge alters sleep,” and it has been suggested that mammalian species which invest in longer sleep times are investing in the immune system, as species with the longer sleep times have higher white blood cell counts.
It has yet to be proven that sleep duration affects somatic growth. One study by Jenni et al. in 2007 recorded growth, height, and weight, as correlated to parent-reported time in bed in 305 children over a period of nine years (age 1–10). It was found that “the variation of sleep duration among children does not seem to have an effect on growth.” It has been shown that sleep—more specifically, slow-wave sleep (SWS)—does affect growth hormone levels in adult men. During eight hours’ sleep, Van Cauter, Leproult, and Plat found that the men with a high percentage of SWS (average 24%) also had high growth hormone secretion, while subjects with a low percentage of SWS (average 9%) had low growth hormone secretion.
There are multiple arguments supporting the restorative function of sleep. The metabolic phase during sleep is anabolic; anabolic hormones such as growth hormones (as mentioned above) are secreted preferentially during sleep. The duration of sleep among species is, in general, inversely related to animal size and directly related to basal metabolic rate. Rats with a very high basal metabolic rate sleep for up to 14 hours a day, whereas elephants and giraffes with lower BMRs sleep only 3–4 hours per day.
Energy conservation could as well have been accomplished by resting quiescent without shutting off the organism from the environment, potentially a dangerous situation. A sedentary nonsleeping animal is more likely to survive predators, while still preserving energy. Sleep, therefore, seems to serve another purpose, or other purposes, than simply conserving energy; for example, hibernating animals waking up from hibernation go into rebound sleep because of lack of sleep during the hibernation period. They are definitely well-rested and are conserving energy during hibernation, but need sleep for something else. Â Rats kept awake indefinitely develop skin lesions, hyperphagia, loss of body mass, hypothermia, and, eventually, fatal septicemia.
Anabolic/catabolic
Non-REM sleep may be an anabolic state marked by physiological processes of growth and rejuvenation of the organism’s immune, nervous, muscular, and skeletal systems (with some exceptions). Wakefulness may perhaps be viewed as a cyclical, temporary, hyperactive catabolic state during which the organism acquires nourishment and reproduces.
Ontogenesis
According to the ontogenetic hypothesis of REM sleep, the activity occurring during neonatal REM sleep (or active sleep) seems to be particularly important to the developing organism (Marks et al., 1995). Studies investigating the effects of deprivation of active sleep have shown that deprivation early in life can result in behavioral problems, permanent sleep disruption, decreased brain mass (Mirmiran et al., 1983), and an abnormal amount of neuronal cell death (Morrissey, Duntley & Anch, 2004).
REM sleep appears to be important for development of the brain. REM sleep occupies the majority of time of sleep of infants, who spend most of their time sleeping. Among different species, the more immature the baby is born, the more time it spends in REM sleep. Proponents also suggest that REM-induced muscle inhibition in the presence of brain activation exists to allow for brain development by activating the synapses, yet without any motor consequences that may get the infant in trouble. Additionally, REM deprivation results in developmental abnormalities later in life.
However, this does not explain why older adults still need REM sleep. Aquatic mammal infants do not have REM sleep in infancy; REM sleep in those animals increases as they age.
Memory processing
Scientists have shown numerous ways in which sleep is related to memory. In a study conducted by Turner, Drummond, Salamat, and Brown, working memory was shown to be affected by sleep deprivation. Working memory is important because it keeps information active for further processing and supports higher-level cognitive functions such as decision making, reasoning, and episodic memory. The study allowed 18 women and 22 men to sleep only 26 minutes per night over a four-day period. Subjects were given initial cognitive tests while well-rested, and then were tested again twice a day during the four days of sleep deprivation. On the final test, the average working memory span of the sleep-deprived group had dropped by 38% in comparison to the control group.
Memory seems to be affected differently by certain stages of sleep such as REM and slow-wave sleep (SWS). In one study cited in Born, Rasch, and Gais, multiple groups of human subjects were used: wake control groups and sleep test groups. Sleep and wake groups were taught a task and were then tested on it, both on early and late nights, with the order of nights balanced across participants. When the subjects’ brains were scanned during sleep, hypnograms revealed that SWS was the dominant sleep stage during the early night, representing around 23% on average for sleep stage activity. The early-night test group performed 16% better on the declarative memory test than the control group. During late-night sleep, REM became the most active sleep stage at about 24%, and the late-night test group performed 25% better on the procedural memory test than the control group. This indicates that procedural memory benefits from late, REM-rich sleep, whereas declarative memory benefits from early, SWS-rich sleep.
Dreaming
Dreaming is the perception of sensory images and sounds during sleep, in a sequence which the dreamer usually perceives more as an apparent participant than an observer. Dreaming is stimulated by the pons and mostly occurs during the REM phase of sleep.
People have proposed many hypotheses about the functions of dreaming. Sigmund Freud postulated that dreams are the symbolic expression of frustrated desires that had been relegated to the unconscious mind, and he used dream interpretation in the form of psychoanalysis to uncover these desires. See Freud: The Interpretation of Dreams.
Freud’s work concerns the psychological role of dreams, which clearly does not exclude any physiological role they may have. It is not ruled out therefore by the increased modern interest in the organization and consolidation of recent memory and experience. Recent research claims that sleep has this overall role of consolidation and organization of synaptic connections formed during learning and experience.
John Allan Hobson and Robert McCarley’s activation synthesis theory proposes that dreams are caused by the random firing of neurons in the cerebral cortex during the REM period. According to this theory, the forebrain then creates a story in an attempt to reconcile and make sense of the nonsensical sensory information presented to it; hence, the odd nature of many dreams.
