The state of sleep is still one of the great curiosities of medical science. Although it has been studied in considerable depth for more than a century, only in the last thirty years or so have scientists been able to make any real impact in finding out what people do with up a third of our lives.
Sleep is a behaviour pattern which humans and many other animals exhibit. Dolphins, for example, appear to sleep with one half of their brain at a time, so that the other half is aware and watchful of danger or threat. The human situation is in marked contrast to this. During scientific observations of the behaviour patterns of sleep, humans show hardly any awareness of their surroundings and some scientists believe we are therefore unconscious of them. Physiologically, the body expends much less energy when it is asleep; muscles relax, blood pressure drops and heart and breathing rates are reduced. However at certain times during sleep the blood circulation to the brain rises and there is also good evidence that certain kinds of brain activity – both electrical and chemical – actually increase. So although we are unaware of thoughts and external stimuli (except for loud noises or certain kinds of touch) the sleeping human brain is in fact quite busy even though it seems largely ‘switched off from the body.
What makes us sleep?
The precise stimuli that make us want to go to sleep, and the ways these stimuli act are not well understood, although it is widely believed that the mechanisms are almost certainly connected with biorhythms that determine our degree of activity during the daylight hours and stimulate the desire to sleep in darkness. Physical activity resulting in ‘tired’ muscles is an obvious cue for sleeping. More interesting, however, is the evidence from research that suggests that in fact it seems to be the brain rather than the remainder of the body that has the most demanding need to recuperate through sleep.
Sleep appears to be a behavioural response to pressures in our environment such as learning and coping with the day. In the past, some psychologists have interpreted sleep as a means of ‘keeping out of trouble’. Their arguments cited young animals (including human babies) who spend most of their lives asleep. While young and dependent on their parents, waking is confined only to the hours of feeding. Sleep provides an effective method of staying out of harm’s way, until the young animal is sufficiently large and well developed to survive on its own. Energy is saved during sleep, although in humans the growth hormone secreted by the pituitary gland is released mainly during sleep; this suggests that, apart from its benefits to mental functioning, sleep is also necessary for bodily growth and repair.
The idea that sleep is a survival mechanism was supported by the fact that animals which sleep usually do so in places where they are reasonably safe from predators, although the logic for this could also apply in reverse: animals that did not sleep in a safe place would not survive long. In biological terms the function of one generation is to ensure the production of the next, and reducing the amount of time when one is exposed to the risks of the environment would appear an effective method of ensuring survival in order to reproduce.
Two kinds of sleep
In 1954 scientists discovered that two patterns of electrical activity could be detected in the neurons (nerve cells) of the sleeping brain. Using an electroencephalograph, they showed that one type of sleep is accompanied by electrical activity of low-frequency but high-voltage, whereas the other shows a high-frequency, low-voltage activity accompanied by rapid movements of the eyes. This latter form of sleep was given the name Rapid-Eye-Movement (abbreviated to REM) sleep, and the former was called slow-wave sleep. The importance of REM sleep was realized when it was discovered that it is usually only during this type of sleep that dreaming occurs. Once we have dozed off, we seem to pass through a regular pattern of about three cycles of alternate slow-wave and REM sleep before we wake up next morning. Each cycle lasts around two to twoand-a-half hours; within a cycle, the period of rapid eye movements is quite short and may last only a few minutes. Sleep researchers know that this is the time when dreams occur because research subjects woken from REM sleep are able to recall and report their dreams vividly and in detail. Not being able to remember your dreams does not mean you had no dreams at all, merely that you did not recall the most recent ones. Although in REM sleep you seem very near waking – because of the brain activity and of the involuntary movements – you are in fact quite deeply asleep. Because of this apparent contradiction REM sleep is also called the paradoxical sleep. The moments just before waking are usually spent in REM dream sleep, which is why we often wake with a distinct, though rapidly fading, memory of a dream.
How much sleep do you need?
The amount of sleep needed by an individual has always been the subject of great controversy. Some people manage with five hours sleep or less each night and suffer no undue effects. Many need seven to eight hours; a few require more. Generally babies and children spend more time sleeping than do adults, and they spend more of it dreaming in REM sleep. In humans there is a four-fold increase in the waking hours between birth and maturity. A baby might be expected to sleep 18 or 20 hours each day and be awake for no more than four or six hours; young adults could spent about seven or eight hours asleep and the remaining 16 or 17 awake; and this might decrease to five or six hours of sleep a night in old age. The major difference between long and short sleepers is that the former spend up to three times more of their night’s sleep in REM, although the amount of slow-wave sleep in the two groups is very similar. This does not appear to be harmful to habitual short sleepers, however. If you find you need ten hours sleep each night, or only five hours, do not worry about it – take whatever amount of sleep your mind and body need.
Lack of sleep and its effects
One might think that sleeping behaviour would be something we could do without, since we can protect ourselves at all times and do not need to remain in a restful state to avoid predators. But experience shows us that sleep is essential to our well-being. The desire to sleep can be supressed for a night or two, for late parties and in emergencies, but it is impossible to stay awake and function normally for periods longer than this. The effects of going without sleep become particularly pronounced after about four days. People deprived of sleep for this long begin to hallucinate and are unable to think coherently or do any kind of work. It is for this reason that sleep deprivation makes such an effective method of torture. Once again importance of REM sleep comes to the fore; it has been found that, if people are deprived of only REM sleep, they suffer as if they did not sleep at all. This has given scientists greater understanding of the need to sleep, and especially why we need to dream.