Electric lighting has changed the way we sleep, encouraging us to lose sleep by staying awake much longer after dark than we otherwise would.
Or maybe not. A new study of three contemporary hunter-gatherer tribes found that they stay awake long after dark and sleep an average of 6.5 hours a night. They also don’t nap much . This suggests the way we sleep may not be that different from our ancient forebears.
Historian A. Roger Ekirch suggested that before electric lighting it was common to sleep in two four-hour segments with an hour or so of wakefulness in between. His theory was based primarily on medieval English texts that refer to “first sleep” and “second sleep” and has other literary support as well. A small study found that subjects settled into the sleep pattern Ekirch predicted when they were in a dark room for 14 hours each night for a month. But the hunter-gatherers don’t sleep this way.
Maybe latitude is an important factor. The hunter-gatherers mentioned above live between 2 and 20 degrees south of the equator whereas England is 52 degrees north of the equator. Maybe two-phase sleep was more common at high latitudes with long winter nights. Of course there are many differences between modern/ancient  hunter-gatherers and medieval Western Europeans besides latitude.
Two studies have found two patterns of how people sleep without electric lights. Maybe electric lights don’t have as much impact on how people sleep as other factors.
Related post: Paleolithic nonsense
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 The study participants were given something like a Fitbit to wear. The article said that naps less than 15 minutes would be below the resolution of the monitors, so we don’t know how often the participants took cat naps. We only know that they rarely took longer naps.
 There is an implicit assumption that the contemporary hunter-gatherers live and, in particular, sleep like their ancient ancestors. This seems reasonable, though we can’t be certain. There is also the bigger assumption that the tribesmen represent not only their ancestors but all paleolithic humans. Maybe they do, and we don’t have much else to go on, but we don’t know. I suspect there was more diversity in the paleolithic era than we assume.