Here’s the story that people like to tell about the way we sleep: Back in the day, we got more of it. Our eyes would shut when it got dark. We’d wake up for a few hours during the night instead of snoozing for a single long block. And we’d nap during the day.
Then—minor key!—modernity ruined everything. Our busy working lives put an end to afternoon naps, while lightbulbs, TV screens, and smartphones shortened our natural slumber and made it more continuous.
Siegel’s team has shown that people who live traditional lifestyles in Namibia, Tanzania, and Bolivia don’t fit with any of these common notions about pre-industrial dozing. “People like to complain that modern life is ruining sleep, but they’re just saying: Kids today!” says Siegel. “It’s a perennial complaint but you need data to know if it’s true.”
Such data have been hard to come by because the devices that we use to measure and record sleep have only been invented in the last 50 years, and those that do so without disturbing the sleepers are just a decade old. So, there’s no baseline for how long people used to sleep before electric lights. Absent that baseline, Siegel’s team did the next best thing: They studied people who live traditional lifestyles, including Hadza and San hunter-gatherers from Tanzania and Nambia respectively, and Tsimane hunter-farmers from Bolivia.
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?
His study provides three clues. First, all three groups wake up before sunrise, in stark contrast to Westerners who typically rouse when it’s already light. Once up, the volunteers got the most light exposure at around 9 a.m.; in the middle of the day, when the sun is at its strongest, they head for shade. Siegel thinks this might explain why people with seasonal affective disorder respond well to bright light, especially in the early morning. “It seems like more than a coincidence that this is when all these groups are getting their maximal light exposure,” he says. “We have lost this exposure by living indoors the way we do.”
Second, the volunteers woke up at virtually the same time every day. “They get up at 7 a.m. today and 7 a.m. tomorrow. The day-to-day variability is almost zero,” says Eus van Someren from the Netherlands Institute for Neuroscience, who was not involved with the study. “This is advice we give to people with insomnia: No matter how much sleep you’ve had, always try to get up at the same time.”
The Hadza, Tsimane, and San were also strongly affected by falling temperature, much more so than failing light. They start to sleep as the night cools and begin waking up at its coldest point. “This suggests that temperature is a very strong and evolutionarily old signal that gets integrated into sleep-regulating systems in the brain, and that we could exploit better,” says van Someren. And as Siegel adds, “This temperature rhythm has been reduced or completely eliminated for most of us by our shelters and heating systems.”
“I think that these three things—sleeping during declining temperature, getting up at the same time of day every day, and exposing yourself to a lot of bright light in the morning—may be key to sound sleep,” says van Someren.
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