Sleep study on modern-day hunter-gatherers dispels notion that we’re wired to need 8 hours a day
Modern life's sleep troubles — the chronic bleary-eyed state that many of us live in — have long been blamed on our industrial society. The city lights, long work hours, commutes, caffeine, the Internet. When talking about the miserable state of our ability to get enough rest, sleep researchers have had a tendency to hark back to a simpler time when humans were able to fully recharge by sleeping and waking to the rhythms of the sun.
It turns out that may not be quite right. In fact, it now appears that our ancestors may not have been getting the doctor-recommended eight hours of sleep, either.
In an intriguing study published in Current Biology this week, researchers traveled to remote corners of the planet to scrutinize the sleep patterns of some of the world's last remaining hunter-gatherers — the Hadza of Tanzania, the San of Namibia, and the Tsimane of Bolivia. Cut off from electricity, media and other distractions, these pre-industrial societies are thought to experience the same sort of natural sleep ancient humans enjoyed more than 10,000 years ago.
Actograms from Hadza, Tsimane and San subjects. Yellow represents the log plot of light level. Red, 1 min intervals with movement. Black, number of movements in each one minute interval. Light blue, rest. And dark blue, sleep. (Cell Biology)
Located in a woodland-savannah habitat 2 degrees south of the equator, the Hazda gather their wild foods each day. The San are not migratory but interact very little with surrounding villages and live as hunter-gatherers. The Tsimane, who live close to the Maniqui River, are hunter-horticulturalist.
Using Actiwatch-2 devices (a kind of a souped-up, medical-grade Fitbit for sleep), researchers recorded the sleeping habits of 94 of these tribespeople and ended up collecting data representing 1,165 days.
What they found was a striking uniformity in their sleep patterns despite their geographic isolation. On average, all three groups sleep a little less than 6.5 hours a night, do not take naps and don't go to sleep when it gets dark. Like many of us, the Hazda, San and Tsimane spent more time in bed — from 6.9 to 8.5 hours — than they do actually sleeping. That computes to a sleep efficiency of between 81 to 86 percent — which is very similar to today's industrial populations.
Jerome Siegel, director of the University of California at Los Angeles's Center for Sleep Research, and his colleagues explained that this suggests that sleep may not be environmental or cultural, but "central to the physiology of humans" living in the tropical latitudes where our species evolved.
"The short sleep in these populations challenges the belief that sleep has been greatly reduced in the 'modern world,' " Siegel said. "This has important implications for the idea that we need to take sleeping pills because sleep has been reduced from its 'natural level' by the widespread use of electricity, TV, the Internet, and so on."
The findings call into question the untold millions that have been spent on research that tries to get to the bottom of why "short" sleepers get only about six hours of sleep a night and the idea that lack of sleep may be a big reason that obesity, mood disorders and other physical and mental ailments have surged in recent decades.
Our ideas about napping may need some revision, too.
Scientists have long documented that people have a tendency to "crash" in energy in the midafternoon, and some have speculated that it's because we've managed to suppress some innate need for a siesta. The new study provides evidence that this is unlikely.
The data from the San in Namibia, for instance, shows no afternoon naps during 210 days of recording in the winter and 10 naps in 364 days in the summer. The findings were similar for the other two tribes, suggesting that napping isn't really a common thing among hunter-gatherers, either. At the high end, the researchers estimated that naps may have occurred on up to 7 percent of winter days and 22 percent of summer days. The researchers noted that the devices they were using weren't great at picking up naps of short durations, so it is possible that some of the study subjects were taking short power naps of less than 15 minutes.
Another fascinating finding from the study had to do with the circadian rhythms related to sunlight. Instead of going to sleep right at dusk, the hunter-gatherers were sleeping an average of 2.5 and 4.4 hours after sunset — well after darkness had fallen. All three tribes had small fires going, but the light itself was much lower than you might get from your average 60-watt bulb. They did, however, have a tendency to wake up around sunrise — an hour before or an hour after, depending on the season and the group.
Siegel and his co-authors investigated this further by looking into the significance of temperature and found that it may play a big role. The research showed that "sleep in both the winter and summer occurred during the period of decreasing ambient temperature and that wake onset occurred near the nadir of the daily temperature rhythm," they wrote.
Relation of sleep to ambient temperature and skin temperature. Sleep offset, averaged across all subjects and all days, consistently occurs near the nadir of daily environmental temperature, in both summer (A) and winter (B). Violet lines, environmental temperature; red lines, abdominal temperature; and blue lines, finger temperature. Black bars, night; orange bars, waking; and blue bars, sleep. Vertical lines at top of the figures indicate light-dark transitions; those at bottom indicate sleep-wake transitions.
It should be noted that the tribespeople studied are different from your average American in a number of respects.
Importantly, very few of the hunter-gatherers suffer from chronic insomnia. There isn't even a word for it in their languages.
In interviews with the researchers conducted through interpreters, 1.5 to 2.5 percent of the study subjects said they had sleep onset or sleep maintenance problems more than once a year, which is far lower than the 10 to 30 percent documented in many countries today. Siegel suggested that this may mean that "mimicking aspects of the natural environment" may be effective in treating some sleep disorders.
The hunter-gatherers are also much healthier. Not a single one is obese, and the mean BMIs among the tribes were between 18.3 and 26.2, which is considered quite slim. They also tend to have lower blood pressure, better heart conditions and higher levels of physical fitness.
Thus comes a critical question. If we can't blame the lack of sleep as causing our obesity, mood disorders and the like, could it it be that the reason we feel so unrested is because of poor health?
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Ariana Eunjung Cha is a national reporter. She has previously served as the Post's bureau chief in Shanghai and San Francisco, and as a correspondent in Baghdad.
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