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 advent 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. 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 of Sleep:
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 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 occurs during sleeping periods.
Recently, researchers have uncovered new evidence supporting the repair and restoration theory, discovering that sleep allows the brain to perform "housekeeping" duties.
In an 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."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 explain.
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 of Sleep:
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 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 of Sleep:
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 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.
While there is 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.
Gallagher, J. (2013, Oct. 17). Sleep 'cleans' the brain of toxins. <i>BBC News.</i> Retrieved from http://www.bbc.co.uk/news/health-24567412
Xie, L., Hongyi, K., Qiwu, X., Chen, M. J., Liao, Y., Thiyagarjan, M., ... Nedergaard, M. (2013). Sleep drives metabolite clearance from the adult brain. <i>Science, 342(6156), 373-377. DOI: 10.1126/science.1241224