From The Lancet Neurology:
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%).
Fun fact: Adult horses only sleep for 3-5 hours per day. And in a herd, there’s usually one “standing watch” for predators while the others sleep.
Steve Parker, M.D.