The next time you are awake in the middle of the night you can thank your distant ancestors, as new research has shown the reason we sleep less as we get older is to help us survive in hostile environments.
The study looked at a group of modern hunter-gatherers in Tanzania, Africa, who have very similar sleeping environments to our early relatives, without synthetic light, temperature control or mattresses.
The team found that when family members of all different ages live together in one place, at least one person is awake at all times to protect against predators.
This isn’t because they have an organised nightwatch system, but because of natural mismatched sleeping patterns across the generations that mean at least one person is always restless.
A feature that would have been particularly useful when a lion might be on the prowl in the darkness.
In fact this natural cycle worked so well for the Hazda tribe that the researchers found that over a three week period of observation there were only 18 minutes when all adults were sound asleep simultaneously.
“And that’s just out of the healthy adults; it doesn’t include children, or people who were injured or sick,” said co-author David Samson.
The results showed that sleep patterns were rarely in synch with some members of the group (between 20 and 30 people) retiring as early as 8pm and waking at 6am while others snoozed until after 8am, not going to bed until 11pm.
In between, they would rouse and get up to smoke, tend to a crying baby, or relieve themselves before going back to bed. But despite these breaks in sleep, the participants didn’t complain of sleep problems, Samson said.
This could have implications for treating older people in Western societies who complain about restless nights.
Professor Charlie Nunn, 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 in which they were beneficial.”
Insomnia is characterised as difficulty getting to sleep or staying asleep for long enough to feel refreshed the next morning. Occasional episodes of insomnia may come and go without causing any serious problems, but for some people it can last for months or even years at a time.