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For years, public health authorities have warned that smartphones, television screens and the hectic pace of modern life are disrupting natural sleep patterns, fueling an epidemic of sleep deprivation. By some estimates, Americans sleep two to three hours fewer today than they did before the industrial revolution.

But now a new study is challenging that notion. It found that Americans on average sleep as much as people in three different hunter-gatherer societies where there is no electricity and the lifestyles have remained largely the same for thousands of years. If anything, the hunter-gatherer communities included in the new study — the Hadza and San tribes in Africa, and the Tsimané people in South America — tend to sleep even less than many Americans.

The findings are striking because health authorities have long suggested that poor sleep is rampant in America, and that getting a minimum of seven hours on a consistent basis is a necessity for good health. Many studies suggest that lack of sleep, independent of other factors like physical activity, is associated with obesity and chronic disease.

Yet the hunter-gatherers included in the new study, which was published in Current Biology, were relatively fit and healthy despite regularly sleeping amounts that are near the low end of those in industrialized societies. Previous research shows that their daily energy expenditure is about the same as most Americans, suggesting physical activity is not the reason for their relative good health.

The prevailing notion in sleep medicine is that humans evolved to go to bed when the sun goes down, and that by and large we stay up much later than we should because we are flooded with artificial light, said Jerome Siegel, the lead author of the new study and a professor of psychiatry at the Semel Institute of Neuroscience and Human Behavior at U.C.L.A.

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But Dr. Siegel and his colleagues found no evidence of this. The hunter-gatherer groups they studied, which slept outside or in crude huts, did not go to sleep when the sun went down. Usually they stayed awake three to four hours past sunset, with no light exposure other than the faint glow of a small fire that would keep animals away and provide a bit of warmth in the winter. Most days they would wake up about an hour before sunrise.

In a typical night, they slept just six and a half hours — slightly less than the average American. In the United States, most adults sleep seven hours or more a night, though a significant portion of the population sleeps less.

“I think this paper is going to transform the field of sleep,” said John Peever, a sleep expert at the University of Toronto who was not involved in the new research. “It’s difficult to envision how we can claim that Western society is highly sleep deprived if these groups that live without all these modern distractions and pressing schedules sleep less or about the same amount as the average Joe does here in North America.”

On its website, the Centers for Disease Control and Prevention calls insufficient sleep a public health problem in America, “which may be caused by broad scale societal factors such as round-the-clock access to technology and work schedules.” The agency says that adults require seven to eight hours of sleep daily, and that a third of Americans typically sleep less than this amount.

In June, two of the leading sleep associations — the American Academy of Sleep Medicine and the Sleep Research Society — issued recommendations stating that adults should sleep seven or more hours on a regular basis. The groups recommended that people who are concerned they are not getting the right amount of sleep consult a health care provider.

“Sleeping less than seven hours per night on a regular basis is associated with adverse health outcomes, including weight gain and obesity, diabetes, hypertension, heart disease and stroke, depression and increased risk of death,” the recommendations stated.

Nathaniel Watson, the president of the American Academy of Sleep Medicine, said that the recommendations were based on a review of 5,000 studies that assessed sleep and disease in humans. He said most of the studies were based on self-reporting of how much people slept, which tends to be an overestimate because people report the overall time they spend in bed — not the amount of time they were technically asleep, which is usually less.

Dr. Watson pointed out that in the new study, the hunter-gatherer societies were found to have a sleep period — meaning the time they were actually in bed — of roughly seven to eight and a half hours, which he said was consistent with his group’s recommendations.

He said the question of how much sleep people require was a delicate one.

“Really it’s just the amount that allows people to wake up feeling refreshed and alert,” he added.

But Dr. Siegel said he worried that putting a number on the amount of sleep people require could push those who get less to resort to using sleeping pills, which carry severe side effects. About 5 percent of Americans take sleeping pills, a percentage that has doubled in the past two decades.

Jim Horne, the director of the Sleep Research Center at Loughborough University in England, called the new study “excellent and very timely,” and he said it suggests that sleep quality is much more important than quantity.

“There is this concern in the Western world that we need more sleep and that if you get less than seven hours you’re liable to suffer from obesity and diabetes and heart disease,” he said. “But the average amount of sleep in these people was well under what is recommended to us as adequate sleep, and these were very healthy people who are not suffering chronic disease and insomnia.”

Among sleep researchers it is widely believed that people sleep differently today than they did 150 years ago. Many argue that the invention of the electric light bulb in the late 1800s — and all the artificially lit environments that followed — dramatically changed our sleep patterns. Exposure to artificial light at night, whether from light bulbs or computer screens, throws off the body’s biological clock, delaying and reducing sleep, experts say.

Some historians have also argued that it is not natural for people to sleep straight through the night. They say that before the introduction of artificial light it was normal for people to sleep in two intervals separated by an hour of wakefulness, a phenomenon known as segmented sleep, or “first” and “second” sleep.

But Dr. Siegel said he always questioned those assertions because there were no rigorous studies of sleep behaviors back then. He and his colleagues decided that one way to get some insight was to study cultures relatively unaffected by artificial light.

Among those they chose to follow were the Hadza people, who spend their days hunting and foraging in northern Tanzania, much as their ancestors have for tens of thousands of years; the San of Namibia, who have lived as hunter-gatherers in the Kalahari for at least 20,000 years; and the Tsimané, a seminomadic group that lives in the Andean foothills of Bolivia, near the farthest reaches of the human migration out of Africa.

Members of the various tribes were fitted with small wristwatchlike devices that tracked their sleep patterns and their exposure to light across the seasons.

The researchers found that in addition to sleeping roughly similar amounts each night, the three groups rarely took naps during the day and did not sleep in two separate intervals at night.

Dr. Siegel said it was remarkable how closely their sleep patterns overlapped despite the distances between them.

“The Hadza and the San live in the area where we know humans evolved, and then the Tsimané live in some sense at the end of the human migration,” he said. “The fact that we see very similar sleep times gives me great confidence that this is how all of our ancestors slept.”

Their sleep did not seem to be problematic. Chronic insomnia, which affects 20 percent to 30 percent of Americans, occurred in just 2 percent of the hunter-gatherers. The San and the Tsimané did not even have a word for it in their languages.

Dr. Siegel said that ambient temperature may be a major factor. The groups did not go sleep at sunset and they did not wake up at sunrise, suggesting that light exposure did not have much influence on their sleep patterns. But they almost always fell asleep as temperatures began to fall at night, and they would wake up right as the temperatures were rising again.

This suggests that humans may have evolved to sleep during the coldest hours of the day, perhaps as a way to conserve energy, Dr. Siegel said. If falling temperatures at night are a signal to our bodies that it is an ideal time to go to sleep, then that could be one reason chronic insomnia is so prevalent in industrialized societies.

“Today we sleep in environments with fixed temperatures, but none of our ancestors did,” Dr. Siegel said. “We evolved to sleep in a natural environment where the temperature falls at night. Whether we can treat insomnia by putting people in an environment where the temperature is modulated in this way is something to be studied in the future.”



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