The amount of sleep a person need depends a lot on his age. We need less sleep, as we grow older. Babies sleep a whole lot - about 16 or 17 hours a day! But a 12-year-old sleeps only 9 hours; a young adult for 7 hours and 20 minutes; and a 65-year-old, for 6 hours.

Skipping one night's sleep makes a person cranky and clumsy. After missing two nights of sleep, a person will have problems thinking and doing things; his brain and body can't do their normal tasks nearly as well. After five nights without sleep, a person will hallucinate. Eventually, it becomes impossible for the brain to give its directions to the rest of the body without sleep

In spite of a century of scientific study of sleep, including three decades of modern intensive research, the function of sleep remains a biological enigma. This is not to say that there is a paucity of theories of sleep function. On the contrary, there are perhaps too many, given the relatively slim body of unambiguous data that can be marshalled in support of any one of them.

One hypothesis is that sleep serves to reverse and/or restore biochemical and/or physiological processes that are progressively degraded during prior wakefulness. For example: repairing tissues and grow new cells. This classical view of sleep function has prevailed over competing hypotheses, largely because it is so intuitively reasonable, and especially in light of the widespread detrimental psychological and behavioural effects that we all experience with a loss of sleep. Also consistent with this hypothesis is increased growth hormone secretion immediately following sleep onset in humans which remains synchronised with sleep even when normal sleep patterns are inverted from nighttime to daytime. This hypothesis is not supported by the finding of a decrease rather than increase in protein synthesis of the whole body during sleep in humans. The decrease in protein synthesis is attributed to sleep being a period of overnight fasting. Consistent with the restorative theory of sleep function are the increased amounts and "intensity" of sleep during a period of sleep recovery after 24 hours of sleep deprivation in humans and most other mammals.

As we go to sleep, our heart rate falls, our blood pressure drops and our breathing becomes slow and regular. After about 20 minutes, our body usually stops shifting around and we settle into a deep sleep called orthodox sleep. During this stage, our muscles are relaxed and our brain activity is slowed. It is during orthodox sleep that most the growth, maintenance, and repair of our body occurs. This sleep is broken down into periods of shallow sleep and REM (rapid eye movement) sleep. There have been numerous speculations on the functions of REM sleep, including stimulation of brain growth, fine tuning of the binocular oculomotor system, consolidation of memory, erasure of inappropriate memories, and the harmless discharge of strong emotions during sleep which would otherwise intrude into waking behaviour. There is limited evidence favouring any one of these hypotheses.

Clearly, more research is needed before firm conclusions can be drawn regarding either the functions of sleep as a whole or of its respective stages, but the fact that we all sleep and that there is such a persistent drive to attain sleep suggests that it serves a vitally important function that enables us to remain alive on earth.