To Sleep Or Not To Sleep: Understanding Sleep From An Evolutionary Perspective
Written by Akshaya Karthikeyan
Edited by Caleigh Donnelly
May 2, 2021
Edited by Caleigh Donnelly
May 2, 2021
An average person sleeps around 26 years of their lifetime. Sleep plays a critical role in development as well as in the maintenance of our physical and mental health. However, not all organisms sleep. Sleep consumes time used for eating and reproducing and increases chances of predation. Given this information, you may be wondering: how did sleep evolve to become such a big part of our lives?
Much of our understanding about the function of sleep comes from observing the consequences of sleep disorders. The findings from sleep deprivation experiments highlight the detrimental effects of the lack of sleep on various bodily functions such as memory and brain plasticity, immune system regulation, insulin production, and many more. However, these experiments fail to address the normal function of sleep in animals. On the other hand, taking an evolutionary perspective on sleep has revealed possible ‘normal’ functions of sleep. One such metabolic advantage of sleeping is defined as resource coordination. Since resources obtained by an organism are limited, certain reactions that are beneficial might never take place if the resources were not redirected at some point in time¹. Sleep may serve as a coordinator for such activities to take place as it helps to distribute resources and conserve energy.
Sleep, by definition, is a behavioral state where a weak stimulus does not elicit a strong response, but a strong stimulus does. This typically means a reduction in physical activity and responsiveness to environmental cues. However, sleep does not look the same in all organisms. Phylogenetic data suggests that sleep has emerged more than once in different groups of organisms at various times in evolutionary history¹. However, the biochemical and regulatory mechanisms of sleep are maintained throughout the different lineages where sleep has evolved¹. For example, melatonin induces sleep in fish, vertebrates, birds, and even flatworms. The common regulatory mechanisms for sleep provide evidence for a common ancestor state of sleep that highlight its normal function. Sleep is usually associated with animals who have a distinguishable central nervous system and a brain. However, since sleep is a behavior that is maintained for a prolonged period of time, non-neural signals must help maintain it. This can be seen in flatworms where endocrine signals are released to maintain sleep³. More surprisingly, sea sponges can also coordinate sleep-like behavior despite their lack of neurons and muscle². This proves that sleep does not have a specific neural network requirement. Observing sleep behavior in other organisms can help reveal the functions.
Ultimately, sleep is a behavior that is commonly found in many animals to serve various functions, including proper development and maintaining homeostasis. Understanding its origins will provide a different perspective in approaching remedies to sleeping disorders and improving lifestyle as well. So the next time you hit the hay, know that sleep is a shared evolutionary behavior that still remains a puzzling mystery to many.
References
Anafi, R.C., Kayser, M.S., & Raizen, D.M. (2019). Exploring phylogeny to find the function of sleep. Nature Reviews Neuroscience, 20(2), 109–116. https://doi.org/10.1038/ s41583-018-0098-9
Omond, S., Ly, L.M.T., Beaton, R., Storm, J.J., Hale, M.W., Lesku, J.A. (2017). Inactivity is nycthemeral, endogenously generated, homeostatically regulated, and melatonin modulated in a free-living platyhelminth flatworm. Sleep 40(10), zsx124. https://doi.org/10.1093/sleep/zsx124
Sakarya, O., Armstrong, K.A., Adamska, M., Wang, I., Tidor, B., Degan, B.M., Oakley, T.H., Kosik, K.S. (2007). A post-synaptic scaffold at the origin of the animal kingdom. PLOS ONE 2(6), e506. https://doi.org/10.1371/journal.pone.0000506