Varying sleep patterns among people of different ages evolved to protect against danger lurking in the shadows, they claim.
For our ancestors who lived and slept in groups thousands of years ago, it paid to ensure that not everyone was deeply asleep while predators lurked in the shadows.
It could be one reason why people tend to experience more restless nights as they get older.
Dr David Samson, one of the researchers from Duke University in North Carolina, said: “The idea that there’s a benefit to living with grandparents has been around for a while, but this study extends that idea to vigilance during night-time sleep.”
The scientists, who call their theory the “poorly sleeping grandparent hypothesis”, carried out a study of the Hadza people, a traditional hunter-gatherer community in Tanzania.
The Hadza exist as humans did for hundreds of thousands of years before the advent of farming.
Typically they live and sleep in groups of 20-30 people.
During the day, men and women go their separate ways to hunt or forage for tubers and berries in the savannah woodlands.
At night, young and old alike sleep together out in the open or in huts made of woven grass and branches.
But not everyone slept at the same time, the researchers found. Older group members in their 50s and 60s generally went to bed earlier and woke up earlier than those in their 20s and 30s.
As a result, it was rare for the whole group to be sleeping at once.
Out of more than 220 hours of observation, the scientists identified only 18 minutes when all the group’s adults were sound asleep at the same time.
On average, more than a third of the group was alert, or dozing very lightly, at any given time.
The findings, published in the journal Proceedings of the Royal Society B, may help explain why the Hadza generally do not post night-time sentinels. They do not need to, say the researchers.
Co-author Professor Charlie Nunn, an expert in evolutionary anthropology at Duke University, said: “Any time you have a mixed-age group population, some go to bed early, some later. If you’re older you’re more of a morning lark.
“If you’re younger you’re more of a night owl.
“A lot of older people go to doctors complaining that they wake up early and can’t get back to sleep. 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.”