Some scientist consider that sleep is an instinct, a basic need for the body and mind to relax and to escape from the responses needed while awake. We become tired in body and mind if we do not sleep, and scientists have proved that when we do sleep the electrical activity of the brain slows down, although it may be stimulated when we dream. One chemical theory is that a substance needed to maintain the waking state becomes exhausted and may be replenished in sleep. A contrary suggestion is that some poisonous substances built up in wakefulness may be destroyed when we go to sleep.
Other theories connect the need for increased wakefulness with the development of the more sophisticated areas of the brain. This could explain why new born babies whose powers of reasoning have not yet developed, spend most of their lives asleep. It has been demonstrated that a particular part of the brain, the reticular formation, if severed, causes continuous sleep. Although we are not sure why we sleep, there is no doubt that we need to do so and so do most other animals.
The pattern of sleep and wakefulness is closely associated with our habits and senses. Animals which depends upon sight for food, shelter and defense, like man, are diurnal. That means they are for the most part active during the day and sleep at night. The amount of sleep needed by a person to remain in full health varies considerably with age, with different individuals, and even perhaps, with race. Pre-school children generally need ten to twelve hours sleep schoolchildren nine to eleven hours and adults seven to nine hours. Adults seem to need progressively less sleep as they grow older, and exceptional cases are known of elderly people who have remained healthy on two to three hours a night. It has been said that the Japanese, both children and adults, sleep less than Europeans, but that may be due to habit rather than to race.