A sound night's sleep grows more elusive as people get older. But what some call insomnia may actually be an age-old survival mechanism, researchers report. A study of modern hunter-gatherers in Tanzania finds that, for people who live in groups, differences in sleep patterns commonly associated with age help ensure that at least one person is awake at all times. The research suggests that mismatched sleep schedules and restless nights may be an evolutionary leftover from a time many, many years ago, when a lion lurking in the shadows might try to eat you at 2 a.m.
"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 nighttime sleep," said study co-author David Samson, who was a postdoctoral fellow at Duke University at the time of the study. The Hadza people of northern Tanzania live by hunting and gathering their food, following the rhythms of day and night just as humans did for hundreds of thousands of years before people started growing crops and herding livestock. The Hadza live and sleep in groups of 20 to 30 people. During the day, men and women go their separate ways to forage for tubers, berries, honey and meat in the savanna woodlands near Tanzania's Lake Eyasi and surrounding areas. Then each night they reunite in the same place, where young and old alike sleep outside next to their hearth, or together in huts made of woven grass and branches. "They are as modern as you and me. But they do tell an important part of the human evolutionary story because they live a lifestyle that is the most similar to our hunting and gathering past," said co-author Alyssa Crittenden, associate professor of anthropology at the University of Nevada, Las Vegas. "They sleep on the ground, and have no synthetic lighting or controlled climate—traits that characterized the ancestral sleeping environment for early humans," Crittenden said.