The human sleep pattern is paradoxical: Sleep is vital for optimal physical and cognitive performance, yet humans sleep the least of all primates; short, high-quality, and flexibly timed sleep likely originated as a response to predation risks

The Human Sleep Paradox: The Unexpected Sleeping Habits of Homo sapiens. David R. Samson. Annual Review of Anthropology  Volume 50, 2021, online on July 13, 2021. https://doi.org/10.1146/annurev-anthro-010220-075523

Abstract: The human sleep pattern is paradoxical. Sleep is vital for optimal physical and cognitive performance, yet humans sleep the least of all primates. In addition, consolidated and continuous monophasic sleep is evidently advantageous, yet emerging comparative data sets from small-scale societies show that the phasing of the human pattern of sleep–wake activity is highly variable and characterized by significant nighttime activity. To reconcile these phenomena, the social sleep hypothesis proposes that extant traits of human sleep emerged because of social and technological niche construction. Specifically, sleep sites function as a type of social shelter by way of an extended structure of social groups that increases fitness. Short, high-quality, and flexibly timed sleep likely originated as a response to predation risks while sleeping terrestrially. This practice may have been a necessary preadaptation for migration out of Africa and for survival in ecological niches that penetrate latitudes with the greatest seasonal variation in light and temperature on the planet.