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

“I have a feeling that this is the role that retired snow scientists play as well….just watching over the tribe in the wee hours.” D. Ferguson

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Researchers studied the mixed-age sleeping habits of the Hadza, a hunter-gatherer community in Tanzania, for insights into human evolution. CreditAlyssa Crittenden

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. The researchers 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.

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