Sleep is a complex behaviour that is an integral part of the body's strategic adaptation to daily changes in light and temperature. Because we lose consciousness so dramatically when we fall asleep, it was erroneously assumed that brain activity ceased in sleep. The presence of vivid dreams made such a simplistic theory unlikely and, during the past fifty years, scientific research on the brain and body has shown sleep to be richly variegated, exquisitely controlled, and essential to life. It is now also clear that sleep is not always benign but has its own built-in propensity for disorder and disease.
The rich variegation of sleep phenomena can already be appreciated in its definition as a behaviour characterized by postural immobility (but with periodic changes in body position and muscle tone), by decreased response to external stimuli (but with marked fluctuations in threshold to response), by selective sensitivity to some stimuli, and by an orderly sequence of electrical and chemical changes in the brain that affect the entire body and greatly alter the mind. Clearly, sleep is an active, global, organismic state requiring central control by the brain and affording the brain and body a wide variety of functional opportunities.
Subjective experience was not the only obstacle to appreciation of the manifold complexity of sleep. Because of our modesty, we do not normally welcome the observation of our sleep. And because we all tend to sleep at the same time, there is no one to watch over those few who are willing to be observed. The development of sleep laboratories in the last half century has begun to counter these trends and to create the detailed picture we have today, but naturalistic studies of sleep are still woefully inadequate.
Sleep laboratory studies
Most sleep laboratories consist of two rooms; one with a bed for the subject, connected via a one-way window and by cables to the other, an instrument room where a technician monitors the sleeping subject (sometimes also by video). Recordings are made of electrical signs from the brain (electroencephalogram or EEG) ; from the eye (electro-oculogram or EOG) ; and from the muscles (electromyogram or EMG). A polygraph is used to keep track (graph) of the several (poly) signals simultaneously. Other important bodily functions, like body temperature, breathing, heart rate, blood pressure, and even sex organ volume, can also be recorded.
A typical night of sleep in an adult human is divided into four or five distinct cycles of body and brain activity. Each cycle begins with a relaxation phase, showing declines in brain wave (EEG) activation, muscle tone (EMG), eye movement (EOG), heart rate, breathing rate, and blood pressure, all of which typically reach a nadir after 45-60 minutes. This relaxation phase then gradually gives way to an activation phase, in which many of the brain and bodily functions resume the high levels of the awake state. In the face of this activation, sleep is maintained by the active suppression of sensory (input) signals and motor (output) commands.
Over the course of the night the length and depth of the relaxation phase (which is called quiet, NREM (non-rapid eye movement), or EEG slow-wave sleep) declines as the duration and intensity of the activation phase (called active, REM or EEG fast-wave sleep) increases. About 70-80% of an average sleep bout of 6.5-8.0 hours consists of NREM sleep, while 20-30% is REM. Other bodily functions which are associated with NREM sleep include the secretion of the hormones regulating growth and sexual maturation. REM sleep is associated with profound muscle relaxation and with sex organ distension, including full erection (and is therefore a built-in test of physiological potency), and a loss of the capacity for internal temperature regulation. The rapid eye movements that give REM sleep its name are not continuous but occur in flurries or clusters, each of which is associated with (sometimes dramatic) increases in the rate, or with irregularity, of heartbeat and breathing. Awakenings which follow these REM clusters are very likely to yield long and detailed reports of dreaming.
Variations in sleep
Sleep varies markedly over the life cycle as well as overnight. New-born infants lack the capacity for long, deep NREM sleep. This only develops, with brain maturation, during childhood and adolescence. But babies have an exaggerated propensity for REM sleep, often entering it directly from waking (so it can easily be observed by curious carers). Since sleep duration is about twice as long in neonates (16 vs. 8 hours) and REM is twice as common (50% vs. 25%), the new-born spends four times longer in REM than does the adult (8 vs. 2 hours). REM sleep declines dramatically as sleep depth increases with brain maturation and the emergence of the adult pattern.
But this is not the end of the dynamism of sleep development. The capacity for deep NREM sleep falls precipitately between ages 30 and 40. This leads to a normal decline in the ability to sustain sleep and to feel deeply rested by it. REM sleep remains relatively stable, but its decline may cause further deterioration of sleep quality after age 60, especially as other medical problems interfere with sleep.
Individuals also show marked differences in sleep behaviour. Most of us lie between two extreme ends of a bell-shaped curve of sleep length and efficiency. At one end are the short-sleepers, who need as few as 3-5 hours, and at the other are long-sleepers, who need 8-11 hours to feel rested and refreshed by sleep. Short-sleepers tend to be energetic, active, and productive, while long-sleepers tend to be lethargic, sedentary, and reflective. Society, with its interest in tight schedules and productivity, is kind to short but merciless to long-sleepers. Long-sleepers are ill-advised to seek professions, like medicine, which greatly curtail sleep.
Even within individuals of a given sleep need and age, sleep varies from night to night, and poor or lost sleep tends to be rapidly compensated. This reciprocal dynamic is dramatically revealed by studies in which one or another sleep phase or time is deliberately altered and the recovery process is monitored.
Much has been learned about sleep from laboratory studies of non-human animals. For example, the diversity of sleep behaviour increases as the brain becomes more and more specialized during evolution. Below the level of the reptiles (who have clear-cut NREM sleep but not REM), it is difficult to distinguish sleep from simple inactivity. REM sleep first appears in birds and then only fleetingly, because while hatchlings have it in abundance, adults have little or none. REM sleep is first clearly and enduringly seen in mammals, suggesting a relationship to the two features which distinguish that class of animal: large, highly developed brains and the capacity for strong internal temperature control.
Brain mechanisms of sleep
There is exquisite control of sleep by the brain. In mammals, sleep is one of the key bodily functions controlled by the body clock in the hypothalamus. By these means it is also tied to the rhythm of body temperature, such that sleep occurs as body temperature falls and waking occurs when body temperature is highest. For most animals, including humans, these peaks in alertness and energy availability occur during the daylight hours, but animals (like rats) that rely more on smell than on vision are active at night and sleep in the daytime. In very hot climates humans may also shift their activity into the darker, cooler night and have a siesta during the forbiddingly hot period of the early afternoon.
The body clock times the occurrence of sleep via its direct nervous connection between the hypothalamus and other subcortical structures in the lower brain. Of particular importance are those collections of brain cells in the brain stem which manufacture and liberate from their endings two brain chemicals, noradrenaline (norepinephrine) and serotonin, which appear to have energizing effects needed for the waking functions of the brain and the body. In order for sleep to occur the activity of these brain cells must be quelled by the mechanism of inhibition. As their activity is more and more completely diminished, another group of cells becomes increasingly active and liberates more and more molecules of another chemical (acetylcholine), which appears to mediate restorative functions throughout the body and the brain. It is the reciprocal interaction of the two cell groups that appears to provide the basis of the cyclic alternation of NREM and REM sleep and their functional differentiation.
Functions of sleep
Sleep is vitally necessary. Recent experiments on the effects of prolonged sleep deprivation give hints as to why even short-term sleep loss is so disabling and why it is so vigorously compensated by the brain. If sleep deprivation is extended beyond two weeks, rats develop a distinctive group of signs that inevitably leads to their demise. Their skin breaks down and they show an increasing craving for food but cannot maintain their body weight no matter how much they eat. At the same time they develop more and more determined heat-seeking behaviour, as they cannot control their body temperatures when exposed to normal variation in environmental temperature. Short of these extreme effects, more modest sleep deprivation has been shown to create a wide variety of difficulties. Taken together these suggest that sleep may normally play an important role in the maintenance of such important bodily functions as the immune response and metabolic balance, as well as such critical mental functions as attentiveness, learning and memory, and emotional equilibrium. Shakespeare may have been correct when he said that sleep ‘knits up the raveled sleeve of care’, but he was underestimating the more active developmental and survival functions of sleep.
— J. Allan Hobson
See also dreaming; electroencephalogram; sleep disorders; snoring.
