For decades, the answer to “how much sleep does an adult need?” has been simple: eight hours. The idea dates back to the mid-19th century when the notion of eight hours of work, eight for recreation and eight for sleep first took hold.

But growing evidence suggests that eight hours may not be the ideal amount of sleep for everyone, as a new narrative review in Brain Medicine reminds us.

To find out how much people naturally sleep, researchers have studied hunter-gatherer communities whose lifestyles are closer to those of our ancestors and who have little or no access to artificial light. In one early study, researchers found that people in Tanzania, Namibia and Bolivia slept for between 5.7 and 7.1 hours a night. These short sleep durations can’t be attributed to traffic noise, social media use or too much artificial light at night.

The short nights also seemed to be enough: people did not make up for them with long daytime naps. Without an alarm clock, they tended to wake close to dawn, regardless of the day of the week.

Exactly how long people are found to sleep in pre-industrialised societies varies depending on how sleep is........

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