Abstract
Sleep is essential to cognitive function and health in humans, yet the ultimate reasons for sleep—i.e. ‘why’ sleep evolved—remain mysterious. We integrate findings from human sleep studies, the ethnographic record, and the ecology and evolution of mammalian sleep to better understand sleep along the human lineage and in the modern world. Compared to other primates, sleep in great apes has undergone substantial evolutionary change, with all great apes building a sleeping platform or ‘nest’. Further evolutionary change characterizes human sleep, with humans having the shortest sleep duration, yet the highest proportion of rapid eye movement sleep among primates. These changes likely reflect that our ancestors experienced fitness benefits from being active for a greater portion of the 24-h cycle than other primates, potentially related to advantages arising from learning, socializing and defending against predators and hostile conspecifics. Perspectives from evolutionary medicine have implications for understanding sleep disorders; we consider these perspectives in the context of insomnia, narcolepsy, seasonal affective disorder, circadian rhythm disorders and sleep apnea. We also identify how human sleep today differs from sleep through most of human evolution, and the implications of these changes for global health and health disparities. More generally, our review highlights the importance of phylogenetic comparisons in understanding human health, including well-known links between sleep, cognitive performance and health in humans.
Highlights
Sleep is essential to cognitive function and health in humans
Samson and Nunn [21] discovered that human sleep duration is extremely different from phylogenetic predictions: our actual sleep duration falls outside the 95% credible interval, suggesting that we can be more than 95% certain that human sleep differs from other primates
To understand the reasons for short human sleep discussed above (Fig. 1), we can turn to comparative variation in mammalian sleep to ask, ‘what are the factors that influence sleep durations across species?’ Are these factors related to the function of sleep, for example involving the brain, or circadian release of growth hormone? Or are ecological factors more informative of sleep durations, perhaps because they constrain how much time is available for sleep? This comparative perspective can help uncover the factors that have led humans to sleep so differently from other primates
Summary
Sleep is essential to cognitive function and health in humans. For example, experiments have shown that sleep is important for working memory, attention, decision-making, and visual-motor performance [1,2,3]. The growing realization that sleep interconnects deeply with many other physiological and cognitive mechanisms suggests that sleep has many functions, including growth and repair of the body (e.g. release of growth hormone, 15), immune function [16, 17], and even adaptive stillness to avoid predation [18, 19] These functions are likely to vary in importance across species, including in humans compared to other primates. A central premise of our article is that human sleep has undergone changes from our primate ancestors [20, 21] These derived characteristics (and their correlates) may hold important clues to understanding the links between sleep, cognitive performance and human health. Another premise is that humans in the developed world sleep differently than our ancestors did [21, 22]. We suggest that sleep deprivation is a largely unrecognized global health problem that may contribute to both infectious and non-infectious disease risks in developing countries, and to health disparities in developed countries
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