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.”
The team asked 94 people from these groups to wear Actiwatch-2 devices, which automatically recorded their activity and ambient-light levels. The data revealedthat 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.
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 Sommeren 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 Sommeren. 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 Sommeren.
posted by f.sheikh