To see how generalizable these results are in other populations, Dr. Samson is planning to perform similar studies in hunter-gatherer societies farther from the Equator, where there is greater variation in light and temperature.
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.
So when the older members of your family shout at you for staying in bed late, just remind them that you're doing it to protect them from lions in the middle of the night. He's now an assistant professor of anthropology at the University of Toronto, in Mississauga. "They sleep on the ground, and have no synthetic lighting or controlled climate - traits that characterized the ancestral sleeping environment for early humans". Average bed- and wake-up times were around 10 p.m. and 7 a.m., respectively.
In between, they would rouse and get up to smoke, tend to a crying baby, or relieve themselves before going back to bed. All the members of the tribe managed to be asleep all at the same time only for 18 minutes, which makes up for only 0.001 percent of the entire monitored period. On median, over a third of the group was either very lightly dozing or even alert at any given time.
"And that's just out of the healthy adults; it doesn't include children, or people who were injured or sick", Samson said.
The Hazda typically don't post sentinels to keep watch throughout the night because they don't need to. They also claim that it might indicate that flexibility and variation are natural properties of the human sleep.
'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 Dr David Samson of the Duke University in Durham, South Carolina.More news: Magnitude 6 quake hits North Korea sparking fears of nuclear bomb
He was a postdoctoral fellow at Duke University at the time of the study.
The researchers found that the misaligned sleep schedules were a byproduct of changing sleep patterns common with age.
He said they had developed their own theory - the poorly sleeping grandmother hypothesis - that groups with a mixture of ages, including the elderly, might have survived better in an evolutionary context.
Since humans change their sleeping behavior as they get older, it is important that tribes should have more elders who can take care of everything and keep an eye on things while youngsters are resting. Neurologists might give you one explanation, to do with a specific cluster of neurons in the brain's anterior hypothalamus called the ventrolateral preoptic nucleus, which help to regulate sleep and die off as you age.
The research could help us better understand sleep problems in older people, too.
If you've ever wondered why the older we get, the worse we sleep, then we've got an answer for you.