As we age, we get poorer and poorer sleep, but that might actually be an evolutionary adaptation that helped humans survive at night by ensuring one person in a community was awake at all times, according to a study of Hadza hunter-gatherers published this week in Proceedings of the Royal Society B. The authors call it sharing "the task of vigilance during sleep" in order to "reduce the dangers of sleep."

Why it matters: Other studies have shown an age-related variation in sleep times, but this is the first to find it in humans, Alyssa Crittenden, author of the study and an associate professor of anthropology at the University of Nevada, Las Vegas told the NYT. Plus, this could explain why human communities historically have slept in mixed-age groups. "We may be looking at just another reason why grandparents were critical in human evolution," Crittenden said.

The evidence: The authors monitored the activity patterns at night of Hadza hunter-gatherers of Tanzania, one of the last hunter-gatherer group in the world. They observed all subjects were simultaneously marked as sleeping for about 18 minutes total over 20 days of observation. There was a median of 8 individuals awake throughout night-time periods. Older subjects (in their 50s and 30s) went to bed earlier and got up earlier than younger participants.

A caveat: There are also environmental factors that affect the way people sleep, which can't be ignored in this study, such as activity level and social needs.