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 travelled 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.
Located in a woodland-savannah habitat two 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. 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, they spent more than that in bed — from 6.9 to 8.5 hours than actually sleeping. That computes to a sleep efficiency of between 81 to 86 per cent — very similar to today’s industrial populations.
Jerome Siegel, director of the University of California at Los Angeles’s Center for Sleep Research, 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.”
Our ideas about napping may need some revision, too.
Scientists have long documented that people have a tendency to “crash” in the mid-afternoon. Some have speculated that’s because we are suppressing an innate need for 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.
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 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 role of temperature and found that temperature may play a big role. “(S)leep 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 tribespeople studied are different from your average American in a number of respects.
Importantly, very few of the hunter-gatherers suffer from chronic insomnia. It isn’t even a word in their languages.
In interviews with the researchers conducted through translators, 1.5 to 2.5 per cent 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 per cent documented in many countries today.
The hunter-gatherers are also much healthier. Not a single one is obese, and 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 be that the reason we feel so unrested is because of poor health?