Sleep function: an evolutionary perspective

Abstract

Prospective epidemiological studies in industrial societies indicate that 7 h of sleep per night in people aged 18 years or older is optimum, with higher and lower amounts of sleep predicting a shorter lifespan. Humans living a hunter-gatherer lifestyle (eg, tribal groups) sleep for 6-8 h per night, with the longest sleep durations in winter. The prevalence of insomnia in hunter-gatherer populations is low (around 2%) compared with the prevalence of insomnia in industrial societies (around 10-30%). Sleep deprivation studies, which are done to gain insights into sleep function, are often confounded by the effects of stress. Consideration of the duration of spontaneous daily sleep across species of mammals, which ranges from 2 h to 20 h, can provide important insights into sleep function without the stress of deprivation. Sleep duration is not related to brain size or cognitive ability. Rather, sleep duration across species is associated with their ecological niche and feeding requirements, indicating a role for wake-sleep balance in food acquisition and energy conservation. Brain temperature drops from waking levels during non-rapid eye movement (non-REM) sleep and rises during REM sleep. Average daily REM sleep time of homeotherm orders is negatively correlated with average body and brain temperature, with the largest amount of REM sleep in egg laying (monotreme) mammals, moderate amounts in pouched (marsupial) mammals, lower amounts in placental mammals, and the lowest amounts in birds. REM sleep might, therefore, have a key role in the regulation of temperature and metabolism of the brain during sleep and in the facilitation of alert awakening.

Figure 1:. Sleep duration in mammals

Graphs show the sleep duration of carnivores, omnivores, and herbivores. Sleep data are compiled from various studies, many of which were done in zoos or laboratories. The red dot highlights data for humans on each graph. Weight is plotted on a logarithmic scale. Log values for weight were used in correlation calculations versus linear values for sleep duration. The sleep durations of carnivores, omnivores, and herbivores differ significantly (p<0·0002; f test). The sleep durations of carnivores are significantly greater than those of herbivores (p<0·001; t test). Sleep amounts in herbivores are negatively correlated with weight (r=−0·8; p<0·001). However, the correlation between weight and sleep time is not significant in carnivores (r=−0·3) or omnivores (r=−0·3).

Figure 2:. REM sleep duration across species

REM sleep time and total sleep time are shown for a selection of mammalian species. In contrast to other placental mammals, dolphins and other cetaceans only have unihemispheric sleep, so sleep time in dolphins is presented in this figure as half of the total 8 h unihemispheric sleep time. This figure was adapted from reference 50, with permission of the American Association for the Advancement of Science. REM=rapid eye movement. Photo credits: platypus, Martin Pelanek/Shutterstock; opossum, Vladislav T Jirousek/Shutterstock; ferret, jurra8/Shutterstock; big brown bat, Jay Ondreicka/Shutterstock; hedgehog, Vy nguyen 2905/Shutterstock; armadillo, Marcelo Morena/Shutterstock; humans, DisobeyArt/Shutterstock; guinea pig, Birute Vijeikiene/Shutterstock; guinea baboon, Nagel Photography/Shutterstock; sheep, Dawn Quadling/Shutterstock; horse, Mary Swift/Shutterstock; giraffe, Nicola_K_photos/Shutterstock; dolphin, Luis Seijido/Shutterstock.

Figure 3:. REM versus non-REM sleep duration across placental mammalian species

REM sleep duration is correlated with non-REM sleep duration across 47 placental mammalian species (r=0·5, p=0·0002). REM sleep and non-REM sleep data are shown in the appendix (pp 1–3). REM=rapid eye movement.

Figure 4:. REM sleep hours per day versus core body temperature in homeotherm orders

Homeotherms with the highest body temperature (birds) have the lowest amount of REM sleep, with this inverse relation between temperature and REM sleep duration continuing across placental, marsupial, and monotreme species. Many bird and placental species have been studied, hence the very small standard error for their mean REM values. Overall, there is a negative correlation of core temperature with REM sleep duration across orders (r=−0·975; p=0·02).^– REM sleep data are shown in the appendix (pp 1–3). REM=rapid eye movement.

Figure 5:. Effect of brain temperature on REM sleep

The decerebrate animal (ie, with the brainstem experimentally disconnected from the forebrain at the midbrain–forebrain junction) cannot regulate body temperature, which then falls towards room temperature. Data from decerebrate domestic cats (Felis catus) show the effect of passive cooling of the pontine brainstem region, the region which is both necessary and sufficient for REM sleep, on the proportion of REM sleep. As core temperature falls, REM sleep increases well beyond the highest amount of REM sleep seen in the intact cat. Vertical lines represent standard errors, and the boxes denote the mean value. Graph drawn from data in reference 80. REM=rapid eye movement.