We humans spend over a third of our lives sleeping. Of course, this curious phenomenon is not unique to us: the entire animal kingdom sleeps. It has been shown that sleep is vital for immune function, memory formation, muscle and bone growth, and overall cognitive function of organisms. You can even die without sleep: scientists have sleep-deprived lab rats to the point of death in just 11 days, from no apparent anatomical causes (Everson et al., 1989). Given its necessity to our lives, it is a wonder that we still do not have a full grasp of its overarching purpose. However, there are many theories, supported by some experimental and observational evidence. As scientists continue to study this mysterious aspect of our physiology, perhaps firm conclusions can be drawn using some ideas within these theories.
One of the oldest postulated purposes of sleep is found in the Inactivity Theory. This idea is based on the evolutionary pressure that originally caused sleep to evolve. The theory goes that organisms which were inactive at night were less likely to die of predation during the dark. Therefore, those organisms who remained still and in one place during the night time had a greater chance of survival, eventually leading to the evolution of the sleep-wake cycle (Brinkman et al., 2020). This is closely related to the Energy Conservation Theory. Under this hypothesis, sleep evolved to prevent a waste of energy. Since organisms are less likely to find food during the night anyway, it would make sense for these organisms to simply slow down their metabolism during the night, and only become active after the sun has risen (Nunez & Lamoreux, 2020).
However, contemporary scientists largely reject the idea that sleep is useful only for energy conservation. Contrary to popular belief, sleep is a very active process, and not simply a period of dull restfulness. As a result, the brain only uses about 25% less energy while asleep compared to when awake (Jessen et al., 2015).
Even if these theories are true, they only explain the evolutionary origin for sleep, as a way to conserve energy and reduce the risk of predation. However, it is possible that sleep took on an unrelated purpose in the millions or billions of years since its inception. Evolution may have found a way to make sleep integral to life itself, instead of just a precautionary mechanism.
One theory that makes such a claim is the Restoration Theory, proposed in 2006 (Ezenwanne, 2011). Sleep may simply be used to repair the damage and disorganization that organisms experience during the day. This includes a very wide range of functions, such as muscle growth, tissue repair, protein synthesis, and release of hormones into the bloodstream. While all of these processes occur during wakefulness, it is possible that the decreased metabolism and brain activity of sleep allows the body to “focus” on such activities. The Restoration Theory also explains why sleep is necessary for general cognitive function. For example, neurons in the brain produce a chemical called β-amyloid while we are awake, as a byproduct of routine activities. Scientists believe that a buildup of β-amyloid and other toxins leads to both a feeling of “tiredness” and a loss of cognitive efficiency. Sleep allows the body to clear this β-amyloid (as well as other waste products) from the brain (Jessen et al., 2015).
The mechanisms by which the brain reduces the concentrations of these waste products is not yet fully clear. However, recent research points to the “glymphatic system,” a specialized form of the lymphatic system within the brain. In 2013, scientists performed an experience that confirmed the role of sleep within the physiology of the glymphatic system. They injected dyes into the cerebrospinal fluid of mice; the researchers could monitor the flow of this dye through glymphatic vessels of the mice’s brains with two-photon microscopy (a fluorescence imaging technique). The researchers measured the flow rate of glymphatic fluid inside the mice’s brains both when they were asleep and when they were awake. They found that the glymphatic vessels expanded when asleep, allowing much more fluid to move through the brain, removing wastes and toxins from neurons and intercellular space (Xie et al., 2013). This provides empirical evidence in support of the Restoration Theory, at least within the brain. The exact mechanisms by which sleep promotes repair or waste management in other organs has not yet been studied extensively.
Another important function of sleep is detailed in the Brain Plasticity Theory. According to this idea, sleep may be vital for changes in the structure and organization of the brain, particularly in regards to the process of learning (Nunez & Lamoreux, 2020). This theory remains largely unsubstantiated by experimental evidence. However, one piece of observational evidence that supports it is the fact that infants sleep 13 to 14 hours a day. It follows that sleep is necessary for brain development and the formation of new connections between neurons. This is why it is critical for middle and high school age children to receive adequate sleep: without it, their brains may not develop to their fullest potential. Furthermore, sleep is known to promote the formation of memories. This has been verified both in experiments with rats solving mazes, as well as humans studying for exams (e.g. Ólafsdóttir et al., 2018). This fits into the Brain Plasticity Theory, as memories are nothing more than a set of neurons firing in a specific sequence: the formation of these sequences, therefore, may hinge on adequate sleep.
The answer to “What is the purpose of sleep” is probably a combination of those four theories. Scientists are only just beginning to comprehend how sleep works and the mechanisms by which it influences us. In the future, understanding the biological functions of sleep more completely might lead to a cure for sleep-related disorders, such as insomnia and narcolepsy. Perhaps researchers might even find a way to artificially promote the effects of sleep on our bodies, without the need to rest for eight hours each day—a supplement or “cure” for sleeping. For now, though, we are left puzzling over this wonder of our physiology.
References
Brinkman J. E., Reddy V., & Sharma S. (2020). Physiology, Sleep. StatPearls [Internet], https://www.ncbi.nlm.nih.gov/books/NBK482512/
Everson, C. A., Bergmann, B. M., & Rechtschaffen, A. (1989). Sleep deprivation in the rat: III. Total sleep deprivation. Sleep, 12(1), 13–21. https://doi.org/10.1093/sleep/12.1.13
Ezenwanne E. (2011). Current concepts in the neurophysiologic basis of sleep; a review. Annals of medical and health sciences research, 1(2), 173–179. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3507109/
Jessen, N. A., Munk, A. S., Lundgaard, I., & Nedergaard, M. (2015). The Glymphatic System: A Beginner's Guide. Neurochemical research, 40(12), 2583–2599. https://doi.org/10.1007/s11064-015-1581-6
Nunez, K. & Lamoreux, K., medically reviewed by Dasgupta, R. (2020). What Is the Purpose of Sleep?. Healthline, https://www.healthline.com/health/why-do-we-sleep
Ólafsdóttir, H. F., Bush, D., & Barry, C. (2018). The Role of Hippocampal Replay in Memory and Planning. Current biology : CB, 28(1), R37–R50. https://doi.org/10.1016/j.cub.2017.10.073
Xie, L., Kang, H., Xu, Q., Chen, M. J., Liao, Y., Thiyagarajan, M., O'Donnell, J., Christensen, D. J., Nicholson, C., Iliff, J. J., Takano, T., Deane, R., & Nedergaard, M. (2013). Sleep drives metabolite clearance from the adult brain. Science (New York, N.Y.), 342(6156), 373–377. https://doi.org/10.1126/science.1241224
Article written by Alex Borengasser
Article edited by Devanandh Murugesan
Graphics by Tiya Shah
Group advised by Lakshmi Sriram
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