Under natural conditions, non-human primates sleep from sunset to dawn, thus about 10–12 hours. Humans might be expected to show this same pattern but a 2015 study of hunter-gatherer groups in South America and Africa concluded that they sleep for 6–8 hours. This finding refutes the common assumption that sleep duration in industrialized societies has declined.
Sleep duration in humans varies by age, with newborns sleeping 12–15 hours, teens 9–10 hours, and adults 6–8 hours. Healthy older adults maintain this duration. The greatest pressure to fall asleep is felt during the day around 3 PM, and at night around 3 AM.
Seven hours of sleep predicts the longest lifespan. Surprisingly, lifespan was reduced more in those who sleep more, rather than those who sleep less, than 7 hours. More sleep is not correlated with better health perhaps because sleep is incompatible with activity and exercise, which are well-known to promote health.
Disappointingly, no evidence has suggested that sleep restriction in people sleeping longer than 7 hours is beneficial. Perhaps the focus should be on sleep quality.
Adapted from “Sleep function: an evolutionary perspective” by Jerome M. Siegel in The Lancet Neurology, October 2022.
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