Theories on Why We Sleep

Woman sleeping

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Sleep has been the subject of speculation and thought since the time of the early Greek philosophers, but only recently have researchers discovered ways to study sleep in a systematic and objective way. The introduction of new technology such as the electroencephalograph (EEG) has allowed scientists to look at and measure electrical patterns and activity produced by the sleeping brain.


While we can now investigate sleep and related phenomena, not all researchers agree on exactly why we sleep. Sleeping patterns tend to follow a fairly predictable schedule and experts agree that sleep plays an essential role in health and wellness. A number of different theories have been proposed to explain the necessity of sleep as well as the functions and purposes of sleep.

The following are three of the major theories that have emerged. 

Repair and Restoration Theory

According to the repair and restoration theory of sleep, sleeping is essential for revitalizing and restoring the physiological processes that keep the body and mind healthy and properly functioning.

This theory suggests that NREM sleep is important for restoring physiological functions, while REM sleep is essential in restoring mental functions.

Support for this theory is provided by 2011 research that shows periods of REM sleep increase following periods of sleep deprivation and strenuous physical activity. During sleep, the body also increases its rate of cell division and protein synthesis, further suggesting that repair and restoration occur during sleeping periods.

In 2013 researchers have uncovered new evidence supporting the repair and restoration theory, discovering that sleep allows the brain to perform "housekeeping" duties.

In the October 2013 issue of the journal Science, researchers published the results of a study indicating that the brain utilizes sleep to flush out waste toxins. This waste removal system, they suggest, is one of the major reasons why we sleep. It's important to realize, however, this study was done on mice and not humans.

"The restorative function of sleep may be a consequence of the enhanced removal of potentially neurotoxic waste products that accumulate in the awake central nervous system," the study's authors explained.

Earlier research had uncovered the glymphatic system, which carries waste materials out of the brain. According to one of the study's authors, Dr. Maiken Nedergaard, the brain's limited resources force it to choose between two different functional states: awake and alert or asleep and cleaning up. They also suggest that problems with cleaning out this brain waste might play a role in a number of brain disorders such as Alzheimer's disease.

Evolutionary Theory

Evolutionary theory, also known as the adaptive theory of sleep, suggests that periods of activity and inactivity evolved as a means of conserving energy. According to this theory, all species have adapted to sleep during periods of time when wakefulness would be the most hazardous. 

Support for this theory comes from the comparative research of different animal species. Animals that have few natural predators, such as bears and lions, often sleep between 12 to 15 hours each day. On the other hand, animals that have many natural predators have only short periods of sleep, usually getting no more than 4 or 5 hours of sleep each day.

Information Consolidation Theory

The information consolidation theory of sleep is based on cognitive research and suggests that people sleep in order to process information that has been acquired during the day. In addition to processing information from the day prior, this theory also argues that sleep allows the brain to prepare for the day to come.

Some 2012 research also suggests that sleep helps cement the things we have learned during the day into long-term memory. Support for this idea stems from a number of sleep deprivation studies demonstrating that a lack of sleep has a serious impact on the ability to recall and remember information.

The Clean-Up Theory

Another major theory suggests that sleep allows the brain to clean itself up. The October 2013 mouse study found that the brain cleans itself of toxins and waste produced during the day while asleep.

Brain cells produce waste products during their normal activities. As we sleep, fluid flow through the brain increases. This acts as something of a waste disposal system, cleansing out the brain of these waste products. 

A Word From Verywell

While there are research and evidence to support each of these theories of sleep, there is still no clear-cut support for any one theory. It is also possible that each of these theories can be used to explain why we sleep.

Sleeping impacts many physiological processes, so it is very possible that sleep occurs for many reasons and purposes. In all likelihood, sleep serves a number of different physiological and psychological purposes including cleaning up brain toxins and consolidating information into memory.

3 Sources
Verywell Mind uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Ezenwanne E. Current concepts in the neurophysiologic basis of sleep; a reviewAnn Med Health Sci Res. 2011;1(2):173–179.

  2. Xie L, Kang H, Xu Q, et al. Sleep drives metabolite clearance from the adult brainScience. 2013;342(6156):373–377. doi:10.1126/science.1241224

  3. Born J, Wilhelm I. System consolidation of memory during sleepPsychol Res. 2012;76(2):192–203. doi:10.1007/s00426-011-0335-6

By Kendra Cherry
Kendra Cherry, MS, is the author of the "Everything Psychology Book (2nd Edition)" and has written thousands of articles on diverse psychology topics. Kendra holds a Master of Science degree in education from Boise State University with a primary research interest in educational psychology and a Bachelor of Science in psychology from Idaho State University with additional coursework in substance use and case management.