Why do we sleep?

          All animals sleep – reptiles, birds, and mammals sleep [1]. In fact, invertebrates like fruit flies, jellyfish and even microscopic creatures such as Hydra and C. elegans sleep [2–5]! This means that sleeping is observed across the evolutionary tree and evolved millions of years ago!

          Sleep can vary greatly – humans sleep 7-9 hours a day [7], bats sleep up to 20 hours [8] while elephants sleep only 4 hours [1]. Some animals such as humans, honey bees, and squirrels are diurnal (active during the day); others such as owls, mice, and coyotes are nocturnal (active during the night). Some marine animals such as dolphins rest half their brain at a time while the other half remains active [1]! Some birds and animals have been observed to drastically change their sleeping patterns during mating season or after giving birth [1]. While diverse, sleep is extremely important in the animal kingdom and all animals display some form of sleep.

          This leads to the question: why do we sleep? On the surface it seems as though sleep should have never evolved as it does not fit the idea of survival of the fittest. Lying unconscious for several hours would leave one vulnerable to predators and would limit time for foraging/hunting food and mating! If sleep can potentially be so detrimental, why did it evolve? The short answer: we don’t know! Although important, scientists are not sure why we need to sleep. However, we do have some theories.

          Some argue that sleep helps conserve our energy. This, however, is incorrect as our energy expenditure is not significantly lower while we sleep [9]. Many theories of why we sleep are based on the restorative and recuperative properties of sleep. We need sleep to consolidate what we’ve learned and solidify our memories as well as to help forge new neural connections in the brain [6,8,9]. It is also thought that sleep helps repair our brain [8]. Sleep is known to help boost our immune response and detoxify the body [9]. While these theories outline the advantages of sleep itself, they do not explain the need for sleep. They are merely the benefits of sleeping and not the cause.

          An older theory called the ‘adaptive theory’ argues that animals sleep in order to avoid danger. As an example, a deer would be vulnerable to predators such as lions and tigers during the night and by sleeping could reduce its vulnerability to predators. So sleep is a behavioral adaptation by animals to avoid predation and hence confers a survival advantage [9,10]. Although this theory explains why animals may need to hide, it fails to explain why we sleep [9]. Furthermore, we know that animals higher up the food chain, such as predators, also sleep. An apex predator is not at risk of predation and hence would have no reason to sleep.

          Despite all of its obvious disadvantages, sleep must be providing some survival advantage to organisms. All organisms occupy a certain niche – which is a physical and environmental region that a species inhabits. As an example, tigers live in forests while lions reside in savannahs. The niche theory proposes that day and night are two distinct and different niches [9]. Thus, species are better suited to thrive in only one of these two niches [9]. Developing features or traits to adapt to the other niche would result in the organism being less adapted to its original niche [9]. Thus, as a result, animals sleep when they are present in the niche they are not suited to. This explains why we sleep, as being awake in another niche would lead us to lose traits required to survive in our original niche [9]. It also explains why sleep is so universally observed.

          Although difficult to prove, I think that the niche theory is by far the most convincing theory we have of why we sleep. The niche theory also raises the very interesting question of whether sleep would have evolved if we did not have day/night cycles. Scientists are constantly researching sleep to better understand this unique phenomenon. Until then, make sure you get your Z’s!

Glossary:

Diurnal animals – refers to animals that are active during the day.

Nocturnal animals – animals that are active during the night.

Survival of the fittest – the idea that organisms which are best suited or adjusted to their environment will survive.

Adaptive theory – a theory which argues that animals sleep in order to avoid danger.

Behavioral adaptation – are actions that animals take in order to survive. Alarm calls, migration and hibernation are examples of this.

Survival advantage – traits that allow animals to better survive or adapt to their environment.

Apex predator – animals that are at the top of the food chain and have no natural predators.

Niche – (also known as ecological niche) is a specific set of environmental conditions required by a particular species or the role/function it performs in the ecosystem.

Niche theory – argues that day and night are niches. Sleep has evolved as a mechanism to avoid the niche that a species is not suited for, while maintaining features and traits required for the niche it’s adapted to.

References:

1.         Joiner, W. J. Unraveling the Evolutionary Determinants of Sleep. Curr. Biol. 26, R1073–R1087 (2016).

2.         Hendricks, J. C. et al. Rest in Drosophila Is a Sleep-like State. Neuron 25, 129–138 (2000).

3.         Nath, R. D. et al. The Jellyfish Cassiopea Exhibits a Sleep-like State. Curr. Biol. 27, 2984-2990.e3 (2017).

4.         Greenwood, V. Sleep Evolved Before Brains. Hydras Are Living Proof. Quanta Magazine https://www.quantamagazine.org/sleep-evolved-before-brains-hydras-are-living-proof-20210518/ (2021).

5.         Raizen, D. M. et al. Lethargus is a Caenorhabditis elegans sleep-like state. Nature 451, 569–572 (2008).

6.         Brown, E. A. 24 Endearing Photos of Animals Sleeping. Photography https://www.nationalgeographic.com/photography/article/pictures-of-animals-sleeping (2018).

7.         Brain Basics: Understanding Sleep | National Institute of Neurological Disorders and Stroke. https://www.ninds.nih.gov/health-information/public-education/brain-basics/brain-basics-understanding-sleep.

8.         Why Did Sleep Evolve? Scientific American https://www.scientificamerican.com/article/why-did-sleep-evolve/ (2013).

9.         Freiberg, A. S. Why We Sleep: A Hypothesis for an Ultimate or Evolutionary Origin for Sleep and Other Physiological Rhythms. J. Circadian Rhythms 18, 2 (2020).

10.       Meddis, R. On the function of sleep. Anim. Behav. 23, 676–691 (1975).

Images (in clockwise order) by Joshua J. Cotten, Andre Blanco, Diana Parkhouse, Chris Curry, Dušan veverkolog,  Jordan Whitt on Unsplash

Edited by Emma Hays