Age-related insomnia may be a throwback to a time when live-in grandparents helped our ancestors stay safe at night, US researchers say.
Varying sleep patterns among people of different ages evolved to protect against danger lurking in the night, they say.
For our ancestors who lived and slept in groups thousands of years ago, it paid to ensure not everyone was deeply asleep.
It could be one reason why people tend to experience more restless nights as they get older.
Dr David Samson, a researcher at 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.
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.
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.
On average, more than one-third of the group was alert or dozing very lightly at any time.
The findings, published in the journal Proceedings of the Royal Society B, might help explain why the Hadza generally do not post night-time sentinels.
Co-author Professor Charlie Nunn said: "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."
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