Sleep Study on Modern-Day Hunter-Gatherers Dispels Popular Notions

Modern life’s sleep troubles -- the chronic bleary-eyed state that many of us live in -- have long been blamed on our industrial society: city lights, long work hours, commutes, caffeine and the Internet. When discussing the widespread difficulty people have in getting enough rest, sleep researchers have often looked back to a simpler time when humans were supposedly able to recharge fully by sleeping and waking according to the rhythms of the sun. It turns out that this assumption 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, researchers travelled to remote corners of the planet to scrutinise 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.

Located in a woodland-savannah habitat two degrees south of the equator, the Hadza 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 and therefore experience a humid climate, are hunter-horticulturalists. Using Actiwatch-2 devices, a medical-grade wristwatch-like device for measuring sleep, researchers recorded the sleeping habits of 94 of these tribespeople and collected data representing 1,165 days.

What they found was a striking uniformity in sleep patterns despite the groups’ geographic isolation. On average, all three groups slept a little less than 6.5 hours a night, did not take regular naps and did not go to sleep when it became dark. Like many people in industrial societies, the Hadza, San and Tsimane spent more time in bed -- from 6.9 to 8.5 hours -- than they actually spent sleeping. That gives a sleep efficiency of between 81 and 86 percent, which is very similar to that of 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 sleep may not be mainly 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 large sums that have been spent on research trying to discover why "short" sleepers get only about six hours of sleep a night. They also challenge the idea that lack of sleep may be a major reason why obesity, mood disorders and other physical and mental ailments have surged in recent decades. Scientists have long documented that people tend to experience a drop in energy in the mid-afternoon, and some have speculated that this is because modern life has suppressed an innate need for a siesta. The new study provides evidence that this is unlikely. Data from the San in Namibia, for instance, showed no afternoon naps during 210 days of recording in the winter and only 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 habit 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 were not very good at detecting naps of short duration, so it is possible that some of the study subjects were taking brief power naps of less than 15 minutes.

Another fascinating finding from the study concerned circadian rhythms related to sunlight. Instead of going to sleep at dusk, the hunter-gatherers slept an average of 2.5 to 4.4 hours after sunset -- well after darkness had fallen. All three tribes had small fires going, but the light itself was much dimmer than that produced by an average 60-watt bulb. They did, however, tend to wake up around sunrise -- an hour before or after, depending on the season and the group. Siegel and his co-authors investigated this further by considering the significance of ambient temperature and found that it may play a major role. The research showed that "sleep initiation occurred during the period of decreasing ambient temperature and that wake onset occurred near the nadir of the daily temperature rhythm".

It should be noted that the tribespeople studied are different from the average American in several respects. Importantly, very few of the hunter-gatherers suffer from chronic insomnia. There is not even a word for it in their languages. In interviews with the researchers conducted through interpreters, only 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 cannot blame lack of sleep for our obesity, mood disorders and similar problems, could the reason we feel so unrested be poor health?