Why Do We Sleep: The Theories, Purpose, and Functions of Sleeping

Restoring the Body and Mind Part of Sleep's Functions

African American woman sleeping in bed
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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? Discover the theories about the purpose and functions of sleeping as a means to restore both body and mind.

Understanding the Purpose of Sleep

There is as yet no universal agreement about the singular 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. In fact, it is likely that further research will reveal other potential purposes to sleeping.

  1. 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. The glymphatic system flushes chemicals from the brain during sleep, including adenosine, the substance largely responsible for increasing levels of sleepiness during wakefulness.

  2. 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. When morning light returns, it is a powerful stimulus for wakefulness, further preserving the evolutionary advantage of responding to the natural day-night cycles.

  1. 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. Though metabolism slows during slow-wave sleep, the brain is extremely active during rapid eye movement (REM) sleep, thus not fully accounting for the conservation.

  1. Other Theories

    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 during REM sleep. It is important for problem-solving. It may allow for novel solutions to be considered as connections are made in the brain. Sleep also helps to strengthen our immune defenses, reducing the risk of infections. Some even argue that REM is critical for the growth and development of brains in infants.

Regardless of its purpose or function (and no doubt there are many), 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.

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