Losing sleep as you age? Blame evolution

Insomnia and waking earlier in later life may be annoying, but it is an evolutionary survival trait

Having trouble getting a full night’s sleep as we age may actually be an evolutionary survival trait, a study of modern hunter-gatherers in Africa has found. Among family members of different ages living together in the tribe, the differences in their sleeping patterns meant that at least one person was awake, or sleeping very lightly, at all times. Mismatched sleep schedules and restless nights may therefore be an evolutionary leftover from a time thousands of years ago when, for example, a lion might be on the prowl at 2am, the study concluded.

A GOOD night’s sleep becomes more elusive with age, but what older people label insomnia may actually be an evolutionary survival trait that helped keep their ancestors alive, a study of modern hunter-gatherers in Africa has found.

When family members of different ages live together, the differences in their sleeping patterns ensures that at least one person is awake, or sleeping very lightly, at all times, the research suggests. Mismatched sleep schedules and restless nights may therefore be an evolutionary leftover from a time thousands of years ago when, for example, a lion might be on the prowl.

“A lot of older people go to their doctor complaining that they wake up early and can’t get back to sleep,” said Charlie Nunn, professor of evolutionary anthropology at Duke University, North Carolina, and the study’s author. “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.

“Any time you have a mixed-age group population, some go to bed early, some later. If you’re in a lighter stage of sleep you’d be more attuned to any kind of threat in the environment.”

Researchers tested their theory – dubbed ‘poorly sleeping grandparent hypothesis’ – on the Hadza people of northern Tanzania. The Hadza live and sleep in groups of 20 to 30 people in the savannah near Tanzania’s Lake Eyasi. They sleep outside next to their hearth, or together in grass huts.

“They live a lifestyle that is the most similar to our hunting and gathering past,” said Alyssa Crittenden, co-author and associate professor of anthropology at the University of Nevada, Las Vegas. As part of the study, 33 men and women aged 20 to 60 agreed to wear a small watch-like device for 20 days, to record their night-time movements.

The results showed that Hadza sleep patterns were rarely in synch, with some members retiring at 8pm and waking at 6am, while others slept until 8am, having gone to bed at 11pm.

In between, many roused from slumber several times during the night. Out of more than 220 total hours of observation there were only 18 minutes when all adults were sound asleep simultaneously. On average, more than a third of the group was alert, or dozing lightly, at any given time.

The researchers believe that for much of human history, living and sleeping in mixed-age groups of people with different sleep habits helped keep a watchful eye through the night. Because of the evolutionary trait, the Hadza people do not need to post sentinels to keep watch, the team found.

The research was published in Proceedings of the Royal Society B.