KELLY MCEVERS, HOST:
So we've heard about the paleo diet. Now let's talk about paleo sleep. The idea is that all of our modern devices are hurting our ability to sleep. But did people really sleep any better without these devices? NPR's Nell Greenfieldboyce reports on what scientists found when they asked that question.
NELL GREENFIELDBOYCE, BYLINE: Jerry Siegel studies sleep at the University of California, Los Angeles. He and his colleagues recently decided to look at how people sleep in three pre-industrial societies. The Tsimane of Bolivia, the San of Namibia and the Hadza of Tanzania.
JERRY SIEGEL: We wanted to know if the sleep amount and the sleep pattern selected by a million years of evolution has been disrupted by modern life.
GREENFIELDBOYCE: He says this is as close as you're going to get to seeing how our ancient ancestors slept.
SIEGEL: The Hadza are maybe the purest hunter-gatherer group in the world. They have no food storage. Every day they get up and they hunt or they gather berries or fruit.
GREENFIELDBOYCE: You might think that no electric lights means that sunset is bedtime, but no. In the journal Current Biology, the researchers say people stayed up for hours after the sun went down and an average night's sleep was less than seven hours.
SIEGEL: If you compare the sleep duration to similar studies done on normal individuals in our society, the amounts we saw in all three groups were at the low end. So they don't sleep more than we do.
GREENFIELDBOYCE: Siegel says people in these traditional communities rarely nap, they generally don't wake up in the middle of the night, but they do sleep more in winter. And it turns out that, throughout the year, what really seems to affect sleep patterns is not light but temperature.
SIEGEL: So sleep is occurring during the night while temperature is falling, and awakening occurs when the temperature hits its minimum, which is usually before sunrise but sometimes after sunrise.
GREENFIELDBOYCE: This has some interesting implications. Siegel says it looks that we evolved to sleep along with this natural cycle of falling and rising temperatures. But if you live in a heated house, that cycle is probably gone.
SIEGEL: The loss of this strong stimulus may be as disruptive to our natural sleep as anything to do with electric lights.
GREENFIELDBOYCE: Now he wants to see whether temperature changes could be used to help people with insomnia. Nell Greenfieldboyce, NPR News.
(SOUNDBITE OF SONG, "I'M ONLY SLEEPING")
THE BEATLES: (Singing) Lying there and staring at the ceiling, waiting for a sleepy feeling. Transcript provided by NPR, Copyright NPR.