Most of our bodily functions and daily processes are well-understood. We have a pretty good grasp of why the heart beats, why we need to breathe and the purpose of our kidneys. The same cannot be said for sleep. Despite spending nearly one-third of our lives asleep, the fundamental function of sleep remains a mystery.
We do understand what happens when we are not able to sleep. There is a decline in our cognitive ability, our mood deteriorates, and if the period of sleeplessness is long enough, psychotic behavior may emerge. Rats will actually die in about three weeks if not allowed to sleep. However, such studies tell us little about the real biological role of sleep.
There have been many proposed explanations for why virtually all creatures on this planet must sleep. Some believe sleep to be an evolutionary step born from the need to stay out of harm's way when it grows dark outside.