Table of ContentsView AllTable of ContentsRestorative TheoryAdaptive TheoryEnergy ConservationBrain PlasticityFrequently Asked Questions
Table of ContentsView All
View All
Table of Contents
Restorative Theory
Adaptive Theory
Energy Conservation
Brain Plasticity
Frequently Asked Questions
Even though it’s something you have to do every day, why we sleep remains a mystery. Many sleep theories abound, yet scientists are far from universal agreement about how to answer the question, “Why do we sleep at night?” Only in the last few decades have they even begun to unravel sleep’s true secrets. At least four common theories are in play, but it remains to be seen which—if any—are correct.
You may look at these theories and the scientific evidence that supports them and wonder why they all can’t, at least in part, be right. They could be, but researchers are continuing to search for “the” core reason for sleep, and many subscribe to a belief expressed in a 1998 paper:
Allen RechtschaffenSleep can be understood as fulfilling many different functions but intuition suggests there is one essential function. The discovery of this function will open an important door to the understanding of biological processes.
Allen Rechtschaffen
Sleep can be understood as fulfilling many different functions but intuition suggests there is one essential function. The discovery of this function will open an important door to the understanding of biological processes.
The sleep cycle involves several stages, but these theories generally focus onrapid-eye movement (REM) sleep—which is when youdream—and the other stages lumped together as non-REM sleep.
The 4 Stages of Sleep
The restorative theory of sleep, first proposed in 2006, is among the more accepted explanations for why people need sleep. It suggests that the purpose of sleep is to store memory and restore our brains and bodies for the next day.
Some of the brain chemicals that accumulate during the day are associated with the plaques that characterizeAlzheimer’s disease, so it’s theorized that the brain-clearing activities of sleep may protect you against Alzheimer’s.
Also called the evolution theory or preservation theory, the original version of this early theory suggested that while humans evolved, hiding out overnight increased the ability to survive.
For our early ancestors, nighttime was dangerous—especially because the predators who hunt at night function better in the dark than humans do—so it made sense to seek a safe refuge. Also, because they couldn’t be out finding food themselves, their bodies slowed down to conserve energy for when they could be active. People who avoided dangers in this way, the theory argued, lived longer and were more likely to reproduce.
Thus, sleep became an adaptive or evolutionary advantage and became part of the neurochemistry of the species. However, most experts reject this idea because sleeping leaves animals (including humans) vulnerable and defenseless, which doesn’t support the idea that sleeping made our ancestors safer.
A flaw in this theory, some experts say, is that sleeping limits productivity, such as finding food and reproducing, so staying awake longer would be an evolutionary benefit. However, no species evolved without a need for sleep, which calls the adaptive benefit into question.
According to some sleep researchers, the daily need for sleep combined with the incentive not to be out in the dark caused us to adapt to function best during daylight, which prevented us from adapting to the dark.
How to Get Better Sleep
Energy Conservation Theory
In a similar vein to aspects of other theories, some experts theorize the primary purpose of sleep is conserving energy. By sleeping, they say, you’re able to spend part of your time functioning at a lower metabolism.
Thatlowers the number of caloriesyou need to eat. For early humans, that extra food requirement could have been the difference between life and death, or survival of the species versus extinction. It was harder to gather food at night, so it made sense to stay hidden then. They also point to the brain’s need to replenish its reserve of glycogen, which is an important fuel.
However, while it’s true that metabolism slows during non-REM, the brain is extremely active during REM sleep, which some say is a strike against the energy conservation theory.
10 Benefits of Sleep
Brain Plasticity Theory
Among the more recent theories deals withbrain plasticity(also called neuroplasticity), which is the brain’s ability to change and adapt in response to experience. It can change both functional aspects (such as re-learning skills in a new area after damage) and structural aspects (such as forming new pathways due to learning).
The brain plasticity theory says that sleep is necessary for the brain to make structural changes. Support for this theory comes from many places.
As in restorative theory, this concept deals with information processing and memory formation. Research suggests that sleep loss leads to less structural plasticity, which can have a negative effect on alertness, cognition, and mood.Sleep deprivationalso compromises memory formation, which is related to learning and plasticity.
It’s believed that the plasticity theory explains why babies and young children require a lot of sleep—they’re learning so much about the world that their brains need more time to process it. Researchers are even trying to promote less sleep interruption for preterm babies in neonatal intensive care, citing studies about sleep’s long-term impact onbrain developmentand plasticity.
Some researchers have even theorized that sleep is the price we pay for brain plasticity. That concept is based on the importance of the processes that occur during sleep to the brain’s ability to adapt and change.
Declining Sleep NeedsNewborns need between 14 and 17 hours of sleep per day. The recommended amount of sleep declines throughout childhood, with teenagers needing between 8 and 10 hours a day.
Declining Sleep Needs
Newborns need between 14 and 17 hours of sleep per day. The recommended amount of sleep declines throughout childhood, with teenagers needing between 8 and 10 hours a day.
Why do we close our eyes when we sleep?
At what stage of sleep can you experience sleep paralysis?
Sleep paralysis, or the feeling that you are conscious and yet unable to control your body, is very common and typically occurs during the transition from REM to being awake.
Why do we dream?
There are a lot of theories explaining why we dream, such as that dreaming aids in memory processing and that it allows us to process emotions.Sigmund Freud believed that dreams express our unconscious desires and deepest wishes.
A Word From Verywell
While it’s a phenomenon we don’t fully understand, sleep is critical to our daily health. Not only is it necessary for restoration and repair, learning and memory, growth and development, and brain plasticity, sleep also helps with problem-solving, a healthy metabolism, blood-sugar and hormone regulation, heart health, and strengthening immunity.With how crucial it is to our survival, it’s no wonder many of us long for more of it.
11 SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Freiberg AS.Why we sleep: A hypothesis for an ultimate or evolutionary origin for sleep and other physiological rhythms.J Circadian Rhythms. 2020;18:2. Published 2020 Mar 30. doi:10.5334/jcr.189Ezenwanne E.Current concepts in the neurophysiologic basis of sleep; a review.Ann Med Health Sci Res; 1(2):173-179.Xie L, Kang H, Xu Q, et al.Sleep drives metabolite clearance from the adult brain.Science. 2013;342(6156):373–377. doi:10.1126/science.1241224Raven F, Van der Zee EA, Meerlo P, Havekes R.The role of sleep in regulating structural plasticity and synaptic strength: Implications for memory and cognitive function.Sleep Med Rev. 2018;39:3-11. doi:10.1016/j.smrv.2017.05.002Park J.Sleep promotion for preterm infants in the NICU.Nurs Womens Health. 2020;24(1):24-35. doi:10.1016/j.nwh.2019.11.004Tononi G, Cirelli C.Sleep and the price of plasticity: From synaptic and cellular homeostasis to memory consolidation and integration.Neuron. 2014;81(1):12-34. doi:10.1016/j.neuron.2013.12.025U.S. Centers for Disease Control and Prevention.About sleep.National Sleep Foundation.Can you really sleep with your eyes open?Rasch B, Born J.About sleep’s role in memory.Physiol Rev. 2013;93(2):681-766. doi:10.1152/physrev.00032.2012Zhang W, Guo B.Freud’s dream interpretation: A different perspective based on the self-organization theory of dreaming.Front Psychol. 2018;9:1553. doi:10.3389/fpsyg.2018.01553National Institutes of Health, National Heart, Lung, and Blood Institute.Sleep deprivation and deficiency.
11 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Freiberg AS.Why we sleep: A hypothesis for an ultimate or evolutionary origin for sleep and other physiological rhythms.J Circadian Rhythms. 2020;18:2. Published 2020 Mar 30. doi:10.5334/jcr.189Ezenwanne E.Current concepts in the neurophysiologic basis of sleep; a review.Ann Med Health Sci Res; 1(2):173-179.Xie L, Kang H, Xu Q, et al.Sleep drives metabolite clearance from the adult brain.Science. 2013;342(6156):373–377. doi:10.1126/science.1241224Raven F, Van der Zee EA, Meerlo P, Havekes R.The role of sleep in regulating structural plasticity and synaptic strength: Implications for memory and cognitive function.Sleep Med Rev. 2018;39:3-11. doi:10.1016/j.smrv.2017.05.002Park J.Sleep promotion for preterm infants in the NICU.Nurs Womens Health. 2020;24(1):24-35. doi:10.1016/j.nwh.2019.11.004Tononi G, Cirelli C.Sleep and the price of plasticity: From synaptic and cellular homeostasis to memory consolidation and integration.Neuron. 2014;81(1):12-34. doi:10.1016/j.neuron.2013.12.025U.S. Centers for Disease Control and Prevention.About sleep.National Sleep Foundation.Can you really sleep with your eyes open?Rasch B, Born J.About sleep’s role in memory.Physiol Rev. 2013;93(2):681-766. doi:10.1152/physrev.00032.2012Zhang W, Guo B.Freud’s dream interpretation: A different perspective based on the self-organization theory of dreaming.Front Psychol. 2018;9:1553. doi:10.3389/fpsyg.2018.01553National Institutes of Health, National Heart, Lung, and Blood Institute.Sleep deprivation and deficiency.
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
Freiberg AS.Why we sleep: A hypothesis for an ultimate or evolutionary origin for sleep and other physiological rhythms.J Circadian Rhythms. 2020;18:2. Published 2020 Mar 30. doi:10.5334/jcr.189Ezenwanne E.Current concepts in the neurophysiologic basis of sleep; a review.Ann Med Health Sci Res; 1(2):173-179.Xie L, Kang H, Xu Q, et al.Sleep drives metabolite clearance from the adult brain.Science. 2013;342(6156):373–377. doi:10.1126/science.1241224Raven F, Van der Zee EA, Meerlo P, Havekes R.The role of sleep in regulating structural plasticity and synaptic strength: Implications for memory and cognitive function.Sleep Med Rev. 2018;39:3-11. doi:10.1016/j.smrv.2017.05.002Park J.Sleep promotion for preterm infants in the NICU.Nurs Womens Health. 2020;24(1):24-35. doi:10.1016/j.nwh.2019.11.004Tononi G, Cirelli C.Sleep and the price of plasticity: From synaptic and cellular homeostasis to memory consolidation and integration.Neuron. 2014;81(1):12-34. doi:10.1016/j.neuron.2013.12.025U.S. Centers for Disease Control and Prevention.About sleep.National Sleep Foundation.Can you really sleep with your eyes open?Rasch B, Born J.About sleep’s role in memory.Physiol Rev. 2013;93(2):681-766. doi:10.1152/physrev.00032.2012Zhang W, Guo B.Freud’s dream interpretation: A different perspective based on the self-organization theory of dreaming.Front Psychol. 2018;9:1553. doi:10.3389/fpsyg.2018.01553National Institutes of Health, National Heart, Lung, and Blood Institute.Sleep deprivation and deficiency.
Freiberg AS.Why we sleep: A hypothesis for an ultimate or evolutionary origin for sleep and other physiological rhythms.J Circadian Rhythms. 2020;18:2. Published 2020 Mar 30. doi:10.5334/jcr.189
Ezenwanne E.Current concepts in the neurophysiologic basis of sleep; a review.Ann Med Health Sci Res; 1(2):173-179.
Xie L, Kang H, Xu Q, et al.Sleep drives metabolite clearance from the adult brain.Science. 2013;342(6156):373–377. doi:10.1126/science.1241224
Raven F, Van der Zee EA, Meerlo P, Havekes R.The role of sleep in regulating structural plasticity and synaptic strength: Implications for memory and cognitive function.Sleep Med Rev. 2018;39:3-11. doi:10.1016/j.smrv.2017.05.002
Park J.Sleep promotion for preterm infants in the NICU.Nurs Womens Health. 2020;24(1):24-35. doi:10.1016/j.nwh.2019.11.004
Tononi G, Cirelli C.Sleep and the price of plasticity: From synaptic and cellular homeostasis to memory consolidation and integration.Neuron. 2014;81(1):12-34. doi:10.1016/j.neuron.2013.12.025
U.S. Centers for Disease Control and Prevention.About sleep.
National Sleep Foundation.Can you really sleep with your eyes open?
Rasch B, Born J.About sleep’s role in memory.Physiol Rev. 2013;93(2):681-766. doi:10.1152/physrev.00032.2012
Zhang W, Guo B.Freud’s dream interpretation: A different perspective based on the self-organization theory of dreaming.Front Psychol. 2018;9:1553. doi:10.3389/fpsyg.2018.01553
National Institutes of Health, National Heart, Lung, and Blood Institute.Sleep deprivation and deficiency.
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