Our sleep patterns inherited from hunter-gatherers, says study

TORONTO (REUTERS) - Tracking members of Tanzania's remote Hadza tribe with fitbit-like devices has uncovered clues about human sleep.

Researchers say the Hadza were perfect test subjects.

"In the West, 99 per cent of every individual that lives in a post-industrial context is exposed to light pollution," said Assistant Professor David Samson, of the University of Toronto Mississauga.

"So the Hadza are exposed to almost no light pollution at all. And finally they are a group that uses foraging to survive, so they're hunter gatherers. They're using the same subsistence strategy that our ancestors used in the same ecology for essentially for 1.8 million years."

Thirty-three adults from the tribe wore actigraphs on their wrists during the trial.

"We found something quite surprising. In fact we found that their sleep was incredibly asynchronous," said Prof Samson, who is from UTM's anthropology department. "So by this I mean that it was very very rare that any of the individuals were asleep all at the same time.

"In fact, (there were) over 200 hours of our studies, or almost 14,000 epochs - that is minutes analysed throughout the nighttime period - and only in 18 of those epochs where all the adults were asleep."

It suggests for the first time that the Sentinel Theory applies to humans - not just animals.

The theory postulates that large groups almost always contain members who are awake, to protect others from danger.

"When you're in REM, you're about as dead to the world as you'll ever be. So it gives you all these cognitive benefits, emotional regulation and memory consolidation, all these really incredible benefits," said Prof Samson.

"But you have to be sleeping securely to be able to go into this stage. So what we think is that having these sentinalised groups was one prerequisite, was one ingredient, that helped humans get better sleep quality throughout evolutionary time."

By showing that sleep variation developed evolutionarily, researchers hope to make clinicians pause before diagnosing patients with sleep disorders.