Our sleep number isn’t naturally 8, study suggests
In remote areas, hunter-gatherers are refreshed on less
Modern life’s sleep troubles — including the chronic, blearyeyed 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 harken back to a simpler time when humans were able to fully recharge by sleeping and waking to the rhythms of the sun.
That may not be quite right. It now appears that our ancestors may not have been getting the eight hours of sleep recommended by doctors, either.
In an intriguing study published in the journal Current Biology, researchers traveled to remote areas 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 preindustrial societies are thought to experience the same sort of natural sleep that ancient humans enjoyed more than 10,000 years ago.
Located in a woodland-savannah habitat 2 degrees south of the equator, the Hadza gather their wild foods each day. The San are not migratory, but they interact very little with surrounding villages and live as hunter-gatherers.
The Tsimane, who live close to the Maniqui River, are hunter horticultural i st.
Using Actiwatch 2 devices (a kind of a souped-up, medicalgrade Fitbit for sleep), researchers recorded the sleeping habits of 94 tribe members and ended up collecting data representing 1,165 days.
What they found was a striking uniformity in subjects’ sleep patterns, despite the geographic isolation. On average, all three groups slept a little less than 6.5 hours a night, did not take naps and did not go to sleep when it got dark. Like many of us, the Hadza, San and Tsimane spent more time in bed — for a total of 6.9 to 8.5 hours — than actually sleeping. That computes to a sleep efficiency of 81 to 86 percent — very similar to today’s industrial populations.
The study’s co-author, 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 in a statement. “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 of dollars 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 might 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 revising, too.
Scientists have long documented that people have a tendency to “crash” in energy in the midafternoon, and some have speculated that it is because we have managed to suppress some innate need for a siesta. The study provides evidence that this is unlikely.
The data from the San in Namibia, for instance, showed 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 is not really a common thing among huntergatherers, 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 were not great at picking up naps of short durations, so it was possible that some of the study subjects were taking 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 huntergatherers were doing so 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.
Siegel and his co-authors investigated this further by looking into the significance of temperature, and found that it might 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.
It should be noted that the tribe members studied are different from your average American in a number of respects.
Importantly, very few of the hunter-gatherers suffer from chronic insomnia. There is no 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 that 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 body mass index readings 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 cannot blame the lack of sleep as causing our collective obesity, mood disorders and the like, could it be that the reason we feel so unrested is because of poor health?
On average, all three groups slept a little less than 6.5 hours a night and did not take naps.