The biological clock that makes us sleepy every 24 hours means that shift workers and those who have just flown to different time zones are often tired and inefficient while they are trying to be alert (see jet lag). If it is a long time since we last slept then that too makes us sleepy. Monotony, warmth, restricted movement, and a sense of waiting for something that cannot happen yet — all of these make us sleepy. A sleeplike state of 'animal hypnosis' can also follow extreme stimulation.
The amount of sleep each species takes is proportional to the need for restoration, i.e. to the waking metabolic rate, but is in part also determined by predator status: those animals who sleep safely sleep longer. Among human beings there are wide variations. A few are happy and healthy with under three hours' sleep a night; a minority of others will take as many as ten hours. In general those who habitually sleep longer have shorter reaction times and higher body temperatures by day. Infants sleep a lot, but the sleep of ageing people becomes more and more broken with the years. A person sleeps less if he gets thin and sleeps longer if he gets fat.
There are two kinds of sleep that in man alternate with each other about every 100 minutes. Orthodox sleep (non-rapid eye movement, or NREM, sleep) occupies 80 per cent of the night, and paradoxical sleep (rapid eye movement, or REM, sleep) about 20 per cent. The amount of paradoxical sleep is greater in the newborn, but in adults is proportional to body weight, and among mental defectives to intelligence. It is diminished by anxiety and by many drugs.
Neither of the two kinds of sleep should be thought of as deeper than the other: they are different. Mental life continues in both, but whereas awakenings from orthodox sleep and questions about preceding mental life generally lead to reports of 'thinking', awakenings from paradoxical sleep are generally followed by detailed descriptions of 'dreaming'. However, the recall of dreams is much diminished if as little as five minutes of orthodox sleep intervenes prior to awakening. In paradoxical sleep most body muscles are profoundly relaxed and many reflexes are lost, the blood flows faster through the brain than during wakeful rest, and in men the penis is erect.
Talking may also occur in both kinds of sleep, and reports made after prompt awakenings show concordance between the words and what was being thought or dreamt about. Sleepwalking (see somnambulism) and 'night terrors' arise from orthodox sleep early in the night, as do most episodes of bedwetting. The shriekings of a night terror often occur with sleepwalking, and liability to them runs in families. They are never remembered in the morning.
Indeed, memory of the events of sleep is always very poor. 'Sleep learning' by means of a tape recorder playing lessons all night is ineffective: although what has been heard while still awake may be remembered, nothing of what was played during sleep will be recalled; to remember we must have paid attention, and in sleep we do not pay attention. Even so, sleep is important for memory. If a list of nonsense words is learned, and memory of them is tested 8 hours or 24 hours later, more of the list will be remembered after 24 hours, given an intervening period of sleep, than after 8 hours without sleep. It seems that memory traces are strengthened during sleep, maybe especially by paradoxical sleep, and, since they presumably depend upon the durable molecules of brain protein, this can be understood.
Protein synthesis is favoured by sleep and so sleep enhances growth and restoration. Tissues such as the skin are restored by growth of new cells, and this growth proceeds faster during sleep. Throughout the body's tissues there are protein molecules being broken down and being synthesized all the time. There is a 24-hour variation in the rate of synthesis, the rate being fastest during the period of rest and sleep. In the cerebral cortex, and the retina, protein synthesis is faster during sleep, and in the anterior pituitary there are more cell divisions. The fact that the balance shifts away from degradation towards greater net protein synthesis is a consequence of a lower rate of cellular work during sleep.
In higher animals there are hormones that reinforce the more fundamental effect of the lower rate of cellular work. In man, growth hormone is specifically released by orthodox sleep with the largest continuous slow electrical brain rhythms ('slow wave sleep': see electroencephalography). Growth hormone promotes protein synthesis. On the other hand, adrenaline (epinephrine) and corticosteroids are hormones which are plentiful during wakefulness and which increase protein breakdown. In the blood during sleep, these latter hormones are diminished, and that means that the growth hormone is even more effective. Slow wave sleep is not merely the time of growth-hormone release, it is the time when responsiveness to meaningful sounds or to an itchy skin is minimal, when the body's oxygen consumption is lowest, and cellular work is lowest, and therefore slow wave sleep is 'worth more' than lighter (more responsive) stages of orthodox sleep. If there is a greater need for restoration, as after sleep deprivation, or after an athlete has trained hard, then the next night there is a higher proportion of slow wave sleep and extra growth hormone.
Sleep deprivation causes sleepiness. It is difficult to keep awake anyone who has been deprived of sleep for 60 hours. Such a person has frequent 'microsleeps' and recurrently fails to notice things he ought to notice, being unable to sustain a high level of attention. Sometimes visual illusions or hallucinations are experienced or the individual becomes paranoid. After about 240 hours there are signs of adaptation to a more uniform but inert and dulled state.
People who complain of lack of sleep (insomnia) actually sleep more than they suppose. Indeed, the most distinguishing feature of their sleep is the degree to which it exceeds their own estimates — but we cannot yet measure its relative restorative value. Complaints are commonest among women, among those of nervous temperament, and among older people, and it is they who account for most of the sleeping pills consumed. Although most sleeping pills today belong to the benzodiazepine class and are safe and effective, prolonged use leads to dependence, and attempts to stop them are accompanied by heightened anxiety, nightmares, and poor sleep for a week or two. Regular physical exercise, a good-quality, firm mattress, a warm but ventilated room, a malted milk drink, and sexual satisfaction at bedtime — all these promote good sleep, but they will not cure everyone's complaints. It has to be accepted that broken sleep is as normal a part of growing older as are grey hairs or wrinkles, though insomnia of sudden onset can be the result of mental depression, an illness amenable to treatment.
Finally, let it be emphasized that sleep is not a slothful habit. Its study as a necessity encompasses the whole functioning of the body and, with the study of dreams, some of the mind's most intriguing qualities.
(Published 1987)
— Ian Oswald
- Bibliography
- Adam, K., and Oswald, I. (1983). 'Protein synthesis, bodily renewal and the sleep–wake cycle'. Clinical Science, 65.
- Kety, S. S., Evarts, E. V., and Williams, H. L. (eds.) (1967). Sleep and Altered States of Consciousness.
- Luce, G. G., and Segal, J. (1970). Insomnia.
- Oram, J., and Barnes, C. D. (eds.) (1980). Physiology in Sleep.
- Oswald, I. (1980). Sleep.
