If you're feeling sleep deprived, you might not be able to blame it on modern society.
A study of hunter-gatherer societies suggests that our prehistoric ancestors slept for about the same number of hours we do today. And, contrary to the claims of siesta aficionados who say that we are biologically wired to sleep in the middle of the day, our ancestors likely didn’t nap.
In a paper released Wednesday in the journal Current Biology, scientists from American, Mexican, and South African universities describe how they monitored the sleep habits of people from three remote hunter-gatherer societies in Africa and Latin America that scientists believe most closely resemble our prehistoric ancestors.
“The question is, do we sleep less than these people,” the study's author, Jerome M. Siegel, who runs a sleep lab at the University of California in Los Angeles, told The Christian Science Monitor.
After studying a total of 94 adults from the Hadza tribe of northern Tanzania, the San in Namibia, and the Tsimane in Bolivia, Dr. Siegel and his team had their answer. “It is absolutely clear that we don’t,” he said.
The researchers also observed that ambient light levels, contrary to popular belief, didn’t seem to influence when study subjects went to sleep or woke.
“Darkness alone doesn’t force sleep and probably never has in humans,” Siegel said.
This insight challenges widespread beliefs about our own sleep, or lack thereof. A host of maladies have been tied to our declining hours of sleep, and TV, the Internet, and city lights have been vilified for turning our civilization into a horde of insomniacs.
And it’s not just us: Anxiety around sleep predates computer screens by centuries, as sleep neuroscientist Jim Horne pointed out in the Telegraph.
Back in 1894, he writes, an editorial in the British Medical Journal claimed, “The subject of sleeplessness is once more under public discussion." It goes on, "The hurry and excitement of modern life is held to be responsible for much of the insomnia of which we hear."
Siegel’s recent findings, the first to measure sleep habits among hunter-gatherers, he says, may help allay some of the persistent anxiety surrounding the "right" amount of sleep people need.
“I’m not saying people shouldn’t worry about getting enough sleep,” Siegel told the Monitor. What his research is showing, he says, is that “amount of sleep is not related to health, necessarily, and that people should not be worried about short sleep that’s unaccompanied by other problems.”
In their paper, Siegel’s team describes fitting adult men and women of the three tribes each with Philips Actiwatches that constantly measured their wrist movements and the levels of light around them.
Over the course of three years, ending early this year, researchers found that their study participants – regardless of culture or environment – slept each night an average of 6.5 hours.
They didn’t take naps during the day, and most had no trouble falling or staying asleep. “Insomnia” is not even a word in their languages, says Siegel.
What’s more, researchers found that sunset didn’t induce sleep, and that sunrise didn’t necessarily wake people up. All the study participants – experiencing nearly identical lengths of night and day – went to sleep several hours after sunset.
Participants' sleep may have been unconnected to the amount of light, but the researchers found another correlation: temperature.
Regardless of the season, participants consistently awoke just as ambient temperatures reached their lowest point. For the Tsimane and Hadza, who live on different continents, that was about an hour-and-a-half before sunrise. For the San in Namibia it was about an hour after the sun rose in the summer.
These findings suggests that temperature, not light, is a regulator of sleep. In modern society, Siegel says, our insulated buildings and artificial temperature-controlled environments could be throwing off our sleep rhythm.
“We don’t experience this substantial drop in temperature that our ancestors experienced,” he explained. “Perhaps for some people and absence of this cue is making a difference.
This research raises other questions about sleep patterns. For one, if our tribal neighbors are sleeping the same number of hours but not complaining about insomnia or other health problems, are they getting better quality sleep?
“One thing we don’t know is what kind of sleep they’re getting,” said Siegel. “That’s the next thing I want to look at.”