Living Another Day, Thanks to Grandparents Who Couldn’t Sleep

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You may not look forward to sleeping less as you get older. But maybe it wouldn’t seem as bad if you knew it once played an important role in human survival.

A new study, published Tuesday in Proceedings of the Royal Society B, suggests that the way sleep patterns change with age may be an evolutionary adaptation that helped our ancestors survive the night by ensuring one person in a community was awake at all times. They called this phenomenon the “poorly sleeping grandparent hypothesis,” suggesting that an older member of a community who woke before dawn might have been crucial to spotting the threat of a hungry predator while younger people were still asleep. It may explain why people slept in mixed-age groups through much of human history.

“We may be looking at just another reason why grandparents were critical in human evolution,” said Alyssa Crittenden, an author of the study and an associate professor of anthropology at the University of Nevada, Las Vegas.

Researchers analyzed the sleep patterns of a society of hunter-gatherers in Tanzania called the Hadza. Thirty-three members of the Hadza community wore small watchlike tracking devices on their wrists for 20 days.

The Hadza sleeping environment may have similarities to that of earlier humans, researchers said. They sleep outdoors or in grass huts in groups of 20 to 30 people without artificially regulating temperature or light. These conditions provide a suitable window to study the evolutionary aspects of sleep.

Out of more than 220 total hours of sleep observation, researchers found only 18 minutes when all adults were sound asleep simultaneously. Typically, older participants in their 50s and 60s went to bed earlier and woke up earlier than those in their 20s and 30s. On average, more than a third of the group was alert, or lightly dozing, at any given time.

Previous studies have observed this age-related variation in sleep times in animals, but this was the first study to find it in humans, Dr. Crittenden said.

The researchers hope these findings will shed new light on age-related sleep disorders and normalize the changes in sleep patterns.

“We have a propensity to overcategorize things as disorders in the West,” said David Samson, an author of the study and an assistant professor of anthropology at the University of Toronto. “It might help elderly individuals to know changes they’re experiencing have an evolutionary reason.”

Lauren Hale, a professor of family population and preventive medicine at the Stony Brook University School of Medicine, who was not involved in the study, said the findings highlighted important social aspects of sleep, which still persist today. But she cautioned that the link might not be entirely biological.

“The variation may be partially explained by genetics,” she said, “but there are environmental conditions too.” As people age, their social needs and level of activity change, potentially affecting their sleep patterns.

More research is needed on the subject, Dr. Crittenden said. But there is evidence of a genetic link, she added, pointing out that sleep quality declined among the older Hadza even while they remained active hunters and gatherers.

To see how generalizable these results are in other populations, Dr. Samson is planning to perform similar studies in hunter-gatherer societies farther from the Equator, where there is greater variation in light and temperature.