It’s tempting to believe that people these days aren’t getting enough sleep, living as we do in our well-lit houses with TVs blaring, cell phones buzzing, and a well-used coffee maker in every kitchen. But new evidence reported in the Cell Press journal Current Biology on Oct 15 shows that three ancient groups of hunter-gatherers—living in different parts of the world without any of those trappings of modern life—don’t get any more sleep than we do.
“The argument has always been that modern life has reduced our sleep time below the amount our ancestors got, but our data indicates that this is a myth,” says Jerome Siegel, leader of the research team and professor of psychiatry at University of California, Los Angeles (UCLA)’s Semel Institute of Neuroscience and Human Behavior, in a release.
Lead author Gandhi Yetish, a PhD candidate at the University of New Mexico, says, “I feel a lot less insecure about my own sleep habits after having found the trends we see here.”
According to the study, people in these three preindustrial societies sleep a little under 6.5 hours a night on average. They don’t take regular naps. They don’t go to sleep at dark, either. In other words, their sleep habits don’t look so different from ours, although they usually do wake up before the sun rises.
“The short sleep in these populations challenges the belief that sleep has been greatly reduced in the ‘modern world,'” Siegel says. “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.”
To get a handle on how people slept before the modern era, Siegel and his colleagues looked to three traditional human hunter-gatherer societies: the Hadza of Tanzania, the San of Namibia, and the Tsimane of Bolivia. The researchers recorded the sleeping habits of 94 individuals around the clock to collect data representing 1,165 days in all.
What they found was a surprising similarity across those three groups. “Despite varying genetics, histories, and environments, we find that all three groups show a similar sleep organization, suggesting that they express core human sleep patterns, probably characteristic of pre-modern-era Homo sapiens,” Siegel says.
Group sleep time averaged between 5.7 and 7.1 hours, with between 6.9 and 8.5 hours between the beginning and end of the sleep period. Those amounts are at the low end of durations reported in “industrial societies.”
One myth dispelled by the results is that in earlier eras people went to bed at sundown. The subjects of the study stayed awake an average of 3 hours and 20 minutes after sunset. “The fact that we all stay up hours after sunset is absolutely normal and does not appear to be a new development, although electric lights may have further extended this natural waking period,” says Siegel, who is also chief of neurobiology research at the Veteran Affairs of Greater Los Angeles Health Care System.
Yetish, who spent 10 months with the Tsimane, says, “There’s this expectation that we should all be sleeping 8 or 9 hours a night and that if you took away modern technology people would be sleeping more. But now for the first time we’re showing that’s not true.”
There is no evidence that these sleep patterns took a toll on people’s health. In fact, extensive studies have found that these groups have lower levels of obesity, blood pressure, and atherosclerosis than people in industrialized societies, and higher levels of physical fitness.
The amount they slept varied with the seasons, with the study’s subjects averaging 6 hours in the summer and just under 7 hours in the winter. Still, they rarely took naps. “There’s this myth that humans used to take daily naps, but that now—because we’re so busy and we can’t get back to our homes—we suppress the naps,” Siegel says. “In fact, napping, is relatively rare in these groups.”
One recent history suggested that humans evolved to sleep in two shifts, a practice chronicled in early European documents. But the people Siegel’s team studied rarely woke for long after going to sleep.
Siegel chalks up the discrepancy between his findings and the historical record to a difference in latitudes. The groups of people studied live near the equator, as did our earliest ancestors; by contrast, early Europeans migrated from the equator to latitudes with much longer nights, which may have altered natural sleeping patterns, he said.
“Rather than saying modern culture has interfered with the natural sleep period, this is a case in which modern culture, with its electric light and temperature control, was able to restore the natural sleep period, which is a single period in traditional humans today and therefore likely in our evolutionary ancestors as well,” Siegel says.
Insomnia was so rare among those studied that the San and the Tsimane do not have a word for the disorder, which affects more than 20% of Americans.
The reason may have to do with sleep temperature. The people studied consistently slept during the nightly period of declining ambient temperature, Siegel found. Invariably, they woke up when temperatures, having fallen all night, hit the lowest point in the 24-hour period. This was the case even when the lowest temperature occurred after daybreak. The pattern resulted in roughly the same wake-up time each morning, a habit long recommended for treating sleep disorders.
“In most modern environments, people are sleeping in a fixed temperature, even if it is reduced from daytime levels,” Siegel says. “It may well be that falling environmental temperature is integral to sleep control in humans.”
The team was surprised to find that all three groups receive their maximal light exposure in the morning. This suggests that morning light may have the most important role in regulating mood and the suprachiasmatic nucleus, a group of neurons that serve as the brain’s clock. Morning light is uniquely effective in treating depression. “Many of us may be suffering from the disruption of this ancient pattern,” Siegel says.
Photo: Two San people in Tsumkwe, Namibia, taken at the start of the study. Photography by Josh Davimes