The Cutting Edge

Our Ancestors Probably Didn’t Get Eight Hours of Sleep a Night, Either

Illustration: Maja Moden

A team of UCLA-led researchers studying sleeping patterns among traditional peoples whose lifestyles closely resemble those of our evolutionary ancestors found that the industrialized world’s sleep habits do not differ much from those of our pre-industrial forebears.

“The argument has always been that modern life has reduced our sleep time below the amount our ancestors got, but our data indicate that this is a myth,” says Jerome Siegel, PhD, professor of psychiatry and biobehavioral sciences.

The researchers monitored sleep patterns among the Hadza, hunter-gatherers who live near the Serengeti National Park in Tanzania; the Tsimane, hunter-horticulturalists who live along the Andean foothills in Boliva; and the San, hunter-gatherers in the Kalahari Desert of Namibia. It is the first study of sleep habits of people who maintain foraging and traditional hunting lifestyles in the present day. Measurements included length of sleep during the summer and winter, body temperatures, environmental temperatures and the amount of light exposure.

One myth dispelled by the results is that people in earlier eras went to bed at sundown. The subjects of the study stayed awake an average of nearly three-and-a-half hours after sunset. “The fact that we all stay up hours after sunset is absolutely normal and does not appear to be a new development,” Dr. Siegel says.

Most of the people studied by Dr. Siegel’s team slept less than seven hours each night, clocking an average of six hours and 25 minutes. The amount is at the low end of sleep averages documented among adults in industrialized societies in Europe and America. The amount they slept also was found to vary with the seasons — less in the summer and more in the winter.

One recent history suggested that humans evolved to sleep in two shifts, a practice chronicled in early European documents. But the people Dr. Siegel’s team studied rarely woke for long after going to sleep. Dr. Siegel chalks up the discrepancy 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 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 percent of Americans. The reason may have to do with sleep temperature. The people studied consistently slept during the nightly period of declining ambient temperature, Dr. Siegel found. Invariably, they woke 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.

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,” Dr. Siegel says.

“Natural Sleep and Its Seasonal Variations in Three Pre-industrial Societies,” Current Biology, November 2, 2015