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Electric Light Means Later Bedtimes

A study finds Argentinian hunter-gatherers without electricity sleep longer than those with power.

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A Toba/Qom baby sleeps in hammock.

It's easy to assume people will sleep more if they don’t have access to electric light. But since nearly everyone in today’s world has power, that’s been hard to test. Until now. New research of powered and unpowered groups in rural Argentina indicates that, yes, people with access to electricity sleep less than people without it.

Researchers found two groups of Toba/Qom hunter-gatherers indigenous to the Chaco region of northeastern Argentina. One group split off from the other in the early 1990s, moving 50 km away to the outskirts of a town with electricity.

Apart from their access to power, the groups remain very similar. They all live off the land and government support because there are few jobs in the region. They still attend joint family parties. And they eat similar diets.

During four weeks in 2012 and 2013, sleep researcher Horacio de la Iglesia, and his colleagues from Harvard and Yale gave Toba/Qom watch-like devices that monitored their activities and ambient light levels. Researchers also brought food and chatted with the locals.

The results, published in the Journal of Biological Rhythms, show that those with power slept an hour less on average than those without. Both groups woke up at about the same time (without alarm clocks), but individuals with electricity had bedtimes that were later and more variable.

Exposure to artificial light in the evenings can throw off the body’s clock, said Dr. Charles Czeisler, a sleep researcher at Harvard Medical School and Brigham and Women’s Hospital who helped conduct the study.

“There’s a surge of energy and alertness that occurs just before dusk,” he said, which provided an evolutionary advantage when we needed to “get our act together” before darkness. But light exposure pushes back this second wind, probably accounting for the later bedtime with electricity.

The study also found that both groups slept about 45 minutes longer during the winter months when their subtropical latitude left them with about 2.5 hours less daylight.

And de la Iglesia’s research shows that most of the Toba/Qom slept through the whole night. However, on moonlit nights, the group without electricity slept more poorly, waking up for a while in the middle of the night, perhaps influenced by the extra light. Historical research by Roger Ekirch at Virginia Tech, has suggested that pre-industrial people slept in two shifts, waking up for a few hours in the middle of the night.

Stanford University sleep researcher Jamie Zeitzer, who was not involved in the study, said he’s not completely convinced that artificial light was the determining factor behind the change in sleep. Access to light at night can increase alertness, he said, but it also can enable people to stay awake longer to do things like read a book or play a game, which could indirectly keep them up later.

The study didn’t address how the difference in sleep might have affected the two groups. Previous research has shown than an hour less sleep a night over a week can reduce performance.

Zeitzer said what really needs to be studied are the long-term effects. Will the group without electricity develop lower levels of diabetes or cardiovascular disease than the others?

De la Igelsia said he hopes to address questions like that in future research.

For himself, having electricity only until 9 p.m. when the local generator went off, made life in Argentina very different than it is at home in Seattle. Going to bed so early, he said, gives morning “a completely new meaning.”

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