The team asked 94 people from these groups to wear Actiwatch-2 devices, which automatically recorded their activity and ambient-light levels. The data revealed that these groups all sleep for nightly blocks of 6.9 and 8.5 hours, and they spend at least 5.7 to 7.1 hours of those soundly asleep. That’s no more than what Westerners who have worn the same watches get; if anything, it’s slightly less.

They don’t go to sleep when it gets dark, either. Instead, they nod off between 2 and 3 hours after sunset, well after it becomes pitch-black. And they napped infrequently: The team scored “naps” as periods of daytime inactivity that lasted for at least 15 minutes, and based on these lenient criteria, the volunteers “napped” on just 7 percent of winter days and 22 percent of summer ones.

The volunteers also slept continuously. They would toss and turn like everyone does, but they almost never woke up for a concerted window in the middle of the night. This contradicts a growing idea, popularized by historian Roger Ekirch, that sleeping in eight-hour chunks is a modern affectation.

Ekirch combed through centuries of Western literature and documents to show that Europeans used to sleep in two segments, separated by an hour or two of wakefulness. Siegel doesn’t dispute Ekirch’s analysis; he just thinks that the old two-block pattern was preceded by an even older single-block one. “The two-sleep pattern was probably due to humans migrating so far from the equator that they had long dark periods,” he says. “The long nights caused this pathological sleep pattern and the advent of electric lights and heating restored the primal one.”

Of course, the Hadza, San, and Tsimane are not ancestral humans. They might live traditional lifestyles, but they are modern people living in today’s world. That’s why Siegel studied all three of them. “If I recorded from just one group, an obvious criticism would be: How do you know this group is typical of our ancestors?” he says. “But given that these groups have different cultures and are isolated from each other, the commonality suggests that this is the basic human wiring, and reflects how humans sleep in the natural environment.”

Horacio de la Iglesia from the University of Washington disagrees that these short sleep durations are a “signature of human ancestral sleep,” noting that some other hunter-gatherer communities sleep for up to 9 hours. And “there is plenty of evidence that industrialization has indeed reduced sleep,” he adds. In one of his own studies, a group of Argentinian hunter-gatherers with access to electric lights got an hour less sleep every day than a neighboring community that relied only on natural lighting. (By contrast, Siegel’s study lacked a similar control group.)

Even if Siegel is right, that doesn’t mean that our sleeping patterns have been unaffected by modern lifestyles. After all, his team found that insomnia, a common affliction of Western society, is almost non-existent in the three groups. Neither the San nor Tsimane even have a word for insomnia in their language. Why?