FIVE THINGS ABOUT SLEEP

1 YOU SNOOZE, YOU LOSE

A good night’s sleep becomes more elusive with age, but what older people label insomnia may actually be an evolutionary survival trait that helped keep their ancestors alive, a new study suggests.

2 ALWAYS SOMEONE ON WATCH

The theory — dubbed “poorly sleeping grandparent hypothesis” — says when family members of different ages live together, the differences in their sleeping patterns ensures that at least one person is awake, or sleeping very lightly, at all times.

3 PUTTING IT TO THE TEST

Researchers tested their theory on the Hadza people of northern Tanzania. The Hadza sleep outside in groups of 20 to 30 people near Tanzania’s Lake Eyasi. As part of the study, 33 healthy men and women agreed to wear a small watch-like device for 20 days, to record their nighttime movements.

4 CONFLICTING SCHEDULES

The results, published in Proceedings of the Royal Society B, showed Hadza sleep patterns were rarely in sync, with some members waking at 6 a.m. while others snoozed until after 8 a.m. In between, many roused from slumber during the night, getting up to smoke, tend to a crying baby, or relieve themselves.

5 NOTHING TO LOSE SLEEP OVER

“A lot of older people go to doctors complaining that they wake up early and can’t get back to sleep,” said author Charlie Nunn, professor of evolutionary anthropology at Duke University, N.C. “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.”