Poor sleep is often regarded as a modern affliction linked to our sedentary lifestyles, electric lighting and smartphones on the bedside table.

However, new research suggests that fitful sleep could be an ancient survival mechanism designed to guard against nocturnal threats. The study, which tracked the sleep patterns of a modern-day hunter-gatherer tribe in northern Tanzania, found that frequent night-time waking and differing sleep schedules between the young and old ensured that there was nearly always at least one tribe member awake.

Over a three-week period, there were only 18 minutes when all 33 tribe members were asleep simultaneously.

David Samson, who was a postdoctoral fellow at Duke University, North Carolina, at the time of the study, said: “What we’re finding in these populations is that the total sleep time is fairly low. In western society, we’re actually getting more secure, decent sleep than hunter-gatherers.”

Despite sleeping less, however, the tribe members were unburdened by “paranoia” about sleep problems and insomnia, which Samson said are common, especially in elderly people, in developed countries.

The study focused on the Hadza people in northern Tanzania, who live and sleep in groups of 20 to 30. During the day, men and women go their separate ways to forage for tubers, berries, honey and meat in the savanna woodlands surrounding Lake Eyasi. They reunite in the evening, typically sleeping all together outside next to their hearth or in huts made of woven grass and branches.



A Hadza man sleeps on an antelope skin in northern Tanzania. Photograph: David Samson/PA

“They tell an important part of the human evolutionary story because they live a lifestyle that is the most similar to our hunting and gathering past,” said Alyssa Crittenden, an associate professor of anthropology at the University of Nevada, Las Vegas, and one of the study’s co-authors. “They sleep on the ground and have no synthetic lighting or controlled climate.”

The researchers tracked the sleep patterns of 33 healthy Hadza men and women, who wore small watch-like devices on their wrists for 20 days, designed to record night-time movements. Typically they woke several times during the night, tossing and turning, getting up to smoke, relieve themselves or tend to crying babies, before nodding off again.

“If you’re in a lighter stage of sleep you’d be more attuned to any kind of threat in the environment,” said co-author Charlie Nunn, professor of evolutionary anthropology at Duke.

Of over 220 total hours of observation, there were just 18 minutes when all adults were sound asleep simultaneously. On average, more than a third of the group was alert, or dozing very lightly, at any given time.

“That figure blew me away,” said Samson. “It normalises this propensity for variation and flexibility in human sleep.”

Previous studies have found similar patterns in birds, mice and other animals, but this is the first time the phenomenon has been observed in humans.

On average, the participants went to bed just after 10pm and woke around 7am. But some settled down for the night as early as 8pm and woke by 6am, with others staying up past 11pm and lying in until after 8am. Older people tended to be “larks”, with the younger participants more likely to stay up late.

The authors claim that the misalignment of sleep schedules of the young and elderly could be an evolutionary adaptation that kept our ancestors safe when sleeping in mixed-age groups.

“A lot of older people go to doctors complaining that they wake up early and can’t get back to sleep,” Nunn said. “But maybe there’s nothing wrong with them. Maybe some of the medical issues we have today could be explained not as disorders, but as a relic of an evolutionary past in which they were beneficial.”