Many theories of sleep have been advanced to explain the temporary loss of consciousness which we know as sleep. The oldest theory is that sleep is induced by a reduction in the blood supply to the brain or at least to conscious centres. This is known as ischemic theory.

Even the ancient Greek physicians were aware that the carotid artery was in a way concerned with the onset of sleep. The name itself expresses this belief. The Greek word 'Karotides' for carotid arteries is derived from karoo which means 'put to sleep.' In modern times, the drowsiness after a meal, presumably due to the diversion of blood from the brain to the digestive organs, is cited in support of the ischemic theory.

Another important theory about sleep is the chemical theory. As a result of experiments in the metabolism of sleeping subjects, it is considered that the fatigue inducing sleep may be a mild form of blood poisoning or toxaemia. This " poisoning" is believed to be brought on by the expenditure of energy during the waking hours.

According to this theory, every contraction of a muscle and every impulse passing through the brain or the nerves breaks down a certain amount of tissue. The debris from broken down tissue is then thrown into the bloodstream. In the waking state, much of the waste from broken down tissue is got rid of through the natural eliminating processes of lungs, kidneys, bowels and skin. But there comes a saturation point when there is such an accumulation of waste that it cannot be disposed of by these processes and it then invades the grey matter of the brain. In such an eventuality, mental and physical altertness are impaired. It is nature's warning that the waste product must be reduced to replenish the lost energy. So we get tired and the urge to get sleep becomes irresistible.

During sleep, the cells and tissues that break down to produce toxic waste become less active and the production of toxic waste is vastly reduced. Simultaneously, constructive activities take place within the body during sleep, which rebuild the broken down tissue. Another theory places a sleeping centre in the hypothalamus. Many of the bodily changes in sleep such as constriction of pupils, reduced frequency of heart beat, increased gastric tone and secretion are manifestations of the activity of hypothalamus nuclei, especially parasympathetic centres. Perhaps some of the sleeping pills affect this centre in the brain.

Although the various theories have certain amount of experimental evidence to support them, none has really solved what is the most mysterious process in our lives. All we know is that sleep substitutes constructive measures for the destructive processes of our waking hours. We cannot live without sleep.