Nobody knows for sure; however, there are many theories.  Below is a very plasable one:\n\nJerome Siegel, of the University of California in Los Angeles, along with colleagues elsewhere, has suggested a third possibility. Writing in a special review of sleep in the current issue of Nature, Dr Siegel says that it may be related to defence against oxidative stress. Smaller animals have higher metabolic rates, and this results in the generation of higher levels of small, highly reactive molecules such as free radicals. Studies on rats show that when they are deprived of sleep, their oxidative stress increases. Dr Siegel proposes that animals with higher metabolic rates need more sleep to interrupt and protect against this kind of oxidative damage, as well as repairing any damage done.
Why do people need sleep?