Our sleep patterns are inherited from hunter-gatherers, says study

Tracking members of Tanzania's Hadza tribe with Fitbit-like devices has uncovered clues about human sleep.

Canadian anthropologist David Samson, an assistant professor from the University of Toronto Mississauga (UTM), says his study of the sleep patterns of the remote tribe indicates human sleep patterns inherited from our hunter-gatherer ancestors.

"In the West, 99 percent of every individual that lives in a post-industrial context is exposed to light pollution. The Hadza are exposed to almost no light pollution at all," Samson told Reuters.

"They're also a group that uses foraging to survive, hunter-gatherers using the same subsistence strategy our ancestors used for 1.8 million years. They're the perfect group to help answer these questions."

Samson and colleagues from Duke University, University of Nevada Las Vegas and Tanzania's University of Dar es Salaam fitted 33 adults with actigraphs—Fitbit-like, wrist-worn devices that measure light and activity.

"We found something quite surprising. Their sleep was incredibly asynchronous, meaning it was very rare that individuals were asleep all at the same time. In 200 hours of our studies, there were only 18 minutes where all the adults were asleep," said Samson.

The findings back the Sentinel Theory, made famous in 1966 by Frederick Snyder, who postulated that in group-dwelling animals various members would be awake at almost all times, to protect others in more vulnerable states, like being asleep.

"When you're in REM, you're dead to the world. It gives you cognitive benefits, emotional regulation and memory consolidation, but you have to be sleeping securely to enter this stage. Having these sentinalized groups helped humans get better sleep quality throughout evolutionary time," said Samson.

He added that this is the first realistic study into human sleep in the context of the Sentinel Theory, and that using actigraphs allowed the first real-life testing of the hypothesis outside of a laboratory setting.

The study also found individual propensities to sleep at particular times were driven by age. Younger Hadza members were mostly "owls" who stayed up late, while elders tended toward "lark" behavior, waking early and often sleeping poorly.

Samson hypothesises that the elders served as sentinels during times of day when others slept, which indicates the need to have people of all ages in any population.

In addition, he thinks it might help change western clinical attitudes to sleep variation.

"If there's one clinical jewel that emerges from this research is that it normalizes variation in sleep patterns in humans. We have a propensity in the West to label anything outside that bell curve as a sleep disorder. This short fragmented sleep in elderly individuals and shift to a more lark-like pattern may have been beneficial in an evolutionary context," he said. — Reuters