Brainwashing. When mice sleep, fluid-filled channels (pale blue) between neurons expand and flush out waste.

Maiken Nedergaard/Jeff Iliff

Chances are, you’re getting enough sleep

No one gets enough sleep in the modern world. That lament has been heard ever since Thomas Edison invented electricity and lit up the night, but it seems to have taken on new urgency in recent years as experts warn us about cellphones, laptops, and e-readers impairing our sleep.

Now, a new study of sleep patterns in hunter-gatherer adults in Africa and South America has found they get no more sleep than people living in industrialized nations—an average of just 6.9 to 8.5 hours every night. What’s more, they rarely nap. Yet, these adults are healthy and don’t feel sleep-deprived, according to a report today in Current Biology. The striking uniformity in the sleep duration and habits of three far-flung groups in Bolivia, Tanzania, and South Africa busts several myths about how much sleep our ancestors got—and what is optimum for modern humans, says Jerome Siegel, senior author of the study and a neuroscientist at the University of California (UC), Los Angeles. Siegel has studied sleep in humans and other mammals for 40 years.

With the invention of electric light bulbs in the 1870s, followed by the development of television, the Internet, and other high tech devices, researchers have argued that the duration of sleep has been shortened from the “natural” levels evolved in our hunter-gatherer ancestors. In the absence of data, some researchers thought our ancestors slept from dusk until dawn and, hence, got 2 to 3 more hours of sleep on average daily than people in industrialized nations. As a result, doctors advised adults to get 8 to 9 hours of sleep per night, on average. (The National Sleep Foundation recommends 7-9 hours for adults.) But this always bothered Siegel: “Before we tell people they need more sleep, we need to make sure this is true.”

Siegel couldn’t go back in time to see how long our ancestors slept, but he could study the sleep patterns of pre-industrialized societies living in South America and Africa. To find out how much they slept each night, he used an Actiwatch 2, a device similar to a Fitbit that measures sleep and waking patterns, as well as light exposure, 24 hours a day for up to 28 days on a single charge. Working with an international team of anthropologists, Siegel and his colleagues strapped Actiwatches onto the wrists of 94 adult members of three remote groups—the Tsimané foragers of Bolivia, the Hadza hunter-gatherers of Tanzania, and the San of Namibia. The devices clocked 1165 total days of sleep data.

When Siegel and his team analyzed the data, the researchers found that the sleep period (total time in bed including brief bouts of waking) in all three groups was similar—between 6.9 to 8.5 hours a night, with actual sleep durations of 5.7 to 7.2 hours a night. Most groups slept an extra hour in winter. They did not go to sleep until 2.5 to 4.4 hours after sunset, nodding off only when temperatures fell and became cooler. They awoke just before dawn—except the San who slept an hour after sunrise in summer—when temperatures were coolest. And they did not wake in the middle of the night to sleep in two phases, as was reported in pre-industrial societies in Europe, says Siegel. They also didn’t report insomnia. (They didn’t even have a word for insomnia.)

It turns out people in all three groups sleep no more than those of us in industrialized nations: A landmark study in 2002 of data from the American Cancer Society on more than 2 million people found that most sleep 6.5 to 7.5 hours a night, on average. “The big story here is that [hunter-gatherer] sleep patterns are not so different from ours,” says co-author Herman Pontzer of Hunter College in New York, a human evolutionary physiologist who studies the Hadza.

The team also learned that cool temperatures may be as important as fluctuations in light to regulate sleep, since these people went to bed well after dark and awoke before sunrise. “While light is important, temperature may be more important from an evolutionary point of view,” says Siegel. This suggests that one bit of common wisdom may be right: To get better sleep, turn down the thermostat or open the window at night.

But unless you are a teenager who needs more sleep for a growing brain, it’s not essential to get 8 hours of sleep per night—unless you feel sleepy during the day. UC San Diego sleep researcher Daniel Kripke—who was not involved in the research—agrees. “I’m inclined to believe that the Old Wives’ tale that people should get 8 hours of sleep is a bunch of baloney.”