You did it today. You've probably done it nearly every day of your life. In fact, you'll spend one third of your life doing it. So why do we sleep? How is it that we slip into restorative repose, only to come back at the proper time?
There is no agreement about the purpose or function of sleep. Though it is extraordinarily commonplace, there is much about sleep that remains a mystery. Only in the last few decades have we even begun to unravel its secrets. There are at least three common theories of why we sleep, but it is unknown which (if any) is correct:
Restorative Theory of Sleep
The restorative theory of sleep is the most accepted explanation for why we sleep. It suggests that sleep restores tissue and prepares our bodies for the next day. This may involve clearing accumulated neurotransmitters from our brain as well as other tissue repair that occurs throughout our bodies.
Adaptive Theory of Sleep
This alternative explanation suggests that sleep increases our ability to survive. As nighttime can be dangerous, especially in animals at risk from predators, it makes sense to seek a safe refuge. By avoiding dangers, the animal lives longer and is more likely to reproduce -- thus, sleep becomes an adaptive advantage.
Energy Conservation Theory
Others theorize that sleep is a means to conserve energy. In a sense, by sleeping we are able to spend part of our time functioning at a lower metabolism, thus our overall caloric needs are reduced. If that time were spent awake, we may not have enough food to survive. It also allows time to create glycogen, an energy store that is used as the brain's fuel reserve.
Sleep seems to have other functions and perhaps these contribute to the phenomenon of sleep. Sleep helps us to learn, refining and consolidating our memories. Sleep helps strengthen our immune defenses. Some even argue that rapid eye movement (REM) sleep is critical for the growth and development of brains in infants.
Regardless of its purpose or function, sleep is clearly one of the singular activities of a lifetime, and is perhaps the one about which we know the least. This leads to opportunities for speculation and philosophical introspection. So if you if you find yourself staring up at the ceiling, waiting once again for sleep to come, ask yourself why we sleep.
Grigg-Damberger, M. "Normal Sleep: Impact of Age, Circadian Rhythms, and Sleep Debt." Continuum. Neurol 2007; 13(3):31-84.
Horne, J. "Why We Sleep." Oxford University Press, Oxford, 1988.