Not Getting 8 Hours of Sleep? Neither Do Hunter-Gatherers

Preindustrial hunter-gatherers did not sleep more than modern-day humans in Western society, a new study shows. Jason Lee / REUTERS

You've probably heard that you need eight hours of sleep to be healthy. You may have also heard that our ancestors slept more than us Westerners, whose sleep habits have been ruined by modern cities, electric lights and TVs.

Both appear to be incorrect. In a study published Thursday in Current Biology, scientists recorded the sleeping patterns of three preindustrial, hunter-gatherer societies in three different environments in Africa and South America. These peoples—the Hadza tribe in Tanzania, the San in Namibia and the Tsimane in Bolivia—had strikingly similar patterns, sleeping between 5.7 and 7.1 hours per night, for an average of 6.4 hours.

That's on the low end of what people in industrial societies get, says study co-author Jerome Siegel, a sleep researcher at UCLA's Semel Institute of Neuroscience and Human Behavior. "We find that humans living in a natural environment do not sleep more than modern-day humans," Siegel says, adding that the findings refute a prevalent idea that the modern life is destroying sleep.

"There's no evidence that we are sleeping fewer hours than we ever have before," says Jim Horne, a scientist at the Sleep Research Centre at England's Loughborough University who wasn't involved in the paper. "The average amount, around seven hours or slightly under, is [probably] no different than it ever was."

The scientists also found that napping isn't an everyday occurrence within the preindustrial groups they studied: On average, they napped on less than 7 percent of the days in winter and 22 percent of days in summer. They did, however, sleep one more hour per night in winter, Siegel says.

The study shows that in these cultures, sleep is heavily influenced by the external environment—it tends to be brought about by darkness and a falling temperature. Re-creating these natural conditions for people with sleeping problems could be beneficial; many people in modern society sleep at a constant temperature, which may not be ideal, Siegel says.

Interestingly, these hunter-gatherers went to sleep about three and a half hours after sundown, perhaps later than expected, and woke up before sunrise, the study found. There is also a very low prevalence of insomnia in these groups, at less than 10 percent.

The researchers also didn't find that the indigenous peoples rarely woke up in the middle of the night and slept through in a single bout. Some historical research shows that sleeping in two periods, so-called segmented sleep, was common in Europe centuries ago. But that pattern likely came about because of low levels of light in the winter, and the single-sleep period found amongst these hunter-gatherers predates that habit, Siegel says.

The scientists tracked the sleep habits of study participants using Actiwatches, devices worn on the wrist that reliably measure sleep by how much the arms are moving. Siegel says that these are less invasive than electroencephalograms that require sleeping in a laboratory, and they have been shown to be quite accurate in a number of validational studies in tracking rest in one's own home.

So what's the takeaway for modern-day humans?

"The major advice one can that if you are sleeping six to seven hours [as objectively measured by a sleep tracker] and feel adequately rested in the day like most Americans, you have nothing to worry about and have little confirmed reason to change," says Dr. Daniel Kripke, a professor emeritus of psychiatry and researcher at the University of California, San Diego. (Most people sleep slightly less than they think, by a few tens of minutes, hence Kripke's qualification about "objective" measurement.) "One could conclude that the average U.S. adult is getting the amount of sleep for which the body is designed and it is healthy."

"I am inclined to conclude that the old wives' tale that people should get eight hours sleep is a bunch of baloney," he adds.

The study doesn't necessarily mean that you need less sleep, though, Siegel says. The real test: Do you feel tired throughout the day? If you do, you likely need more sleep. If you feel you're getting a lot of rest—say, eight hours or more—and continue to feel sleepy, it's important to see a specialist, he says.

David Dinges, a sleep researcher at the University of Pennsylvania School of Medicine who wasn't involved in the study, was a little bit more hesitant to apply the study's findings to people in Western society. Certainly we face dissimilar and perhaps more nagging stressors than our ancestors did: different cognitive demands, more time pressures and a more harried lifestyle. This might make our sleep needs different from those living in hunter-gatherer societies, he says. But researchers simply don't know yet.

Dinges's work has shown that people who sleep seven or more hours have a lowered risk of obesity, metabolic syndrome, accidents and other health problems. Sleeping eight hours or more didn't seem to do much to improve health. But the research is less conclusive when it comes to six to seven hours of sleep per night. Some people appear to do well with this amount, while others do seem to need more.

"We can say absolutely that less than six hours of sleep is problematic in industrial societies," Dinges says. "I don't think this study is a radical challenge to the sleep need but an added piece of data to show that sleep may be manageable even if you're a little short, in the six to seven range."

All of the researchers agree that hypnotic sleeping pills like Ambien and Lunesta should be taken only as a last resort, and that sleeping pills have been tied to a shorter life span. Kripke suggests that the "myth" of the eight-hour sleep cycle has been used by sleeping pill companies to help sell their products.