Anthropologists say in ancient times, with predators close by, staying up at night – and the ability to wake up easily – was the difference between life and death. Picture: Flickr.com

Older people unable to get a proper night’s sleep may blame stress, their medication or even their spouse.

But it may be our caveman past that’s behind the struggle to drift off after we reach a certain age. Anthropologists say in ancient times, with predators close by, staying up at night – and the ability to wake up easily – was the difference between life and death.

A US study tested this theory on tribes in Tanzania, finding that the sleeping patterns of older members helped ensure a "sentinel" to protect the group at night.

Charlie Nunn, professor of evolutionary anthropology at Duke University, 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."

The scientists studied the Hadza people of northern Tanzania, who live by hunting and gathering their food at night.

They asked 33 men and women aged 20 to 60 to wear a device for 20 days, which recorded their movements.

The disturbed sleeping patterns of the older members meant that one or more person was awake or in the light stages of sleep for 99.8 percent of the time.

More than a third of the group were alert, or dozing, at any given time, Proceedings of the Royal Society B reports.