Sleep Disorders Importance of Body and Mind Restoration During Sleep By Brandon Peters, MD facebook twitter Brandon Peters, MD, is a board-certified neurologist and sleep medicine specialist. Learn about our editorial process Brandon Peters, MD Medically reviewed by Medically reviewed by Jason DelCollo, DO on August 05, 2020 Jason DelCollo, DO, is board-certified in family medicine and on the faculty of Philadelphia College of Osteopathic Medicine. Learn about our Medical Review Board Jason DelCollo, DO on August 05, 2020 Print Verywell / Hetal Rathod Though it is extraordinarily commonplace, there is much about sleep that remains a mystery. And, while we all spend one-third of our lives doing it, there is still no universal agreement regarding the singular purpose or function of sleep. Only in the last few decades have we even begun to unravel sleep's true secrets. However, there are at least three common theories as to why we sleep, but it is unknown which (if any) are in fact correct. Restorative Theory 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 repairs that occur throughout our bodies. More specifically, the glymphatic system flushes chemicals from the brain during sleep, including adenosine, the substance largely responsible for increasing levels of sleepiness during wakefulness. Adaptive Theory 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. 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, therefore not fully accounting for the conservation. A Word From Verywell Despite it being a phenomenon that we may not fully comprehend, sleep is critical to our daily health. Not only is it refreshing, but sleep also helps with problem-solving, finding solutions and making connections, strengthening immunity, reducing the risk of infection, growing and developing, learning, and forming memories. No wonder we all pine for that perfect night of it! Was this page helpful? Thanks for your feedback! Tossing and turning night over night can have a big impact on your quality of life. Our free guide can help you get the rest you need. Sign up for our newsletter and get it free. Sign Up You're in! Thank you, {{form.email}}, for signing up. There was an error. Please try again. What are your concerns? Other Inaccurate Hard to Understand Submit Article Sources Verywell Health 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. 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.