A UCLA-led team of researchers studied sleeping
patterns among traditional peoples whose lifestyles
closely resemble those of our evolutionary
ancestors.
What the team found among the Hadza of Tanzania, the
San of Namibia and the Tsimane of Bolivia challenges
conventional wisdom about the sleeping habits of
pre-industrial humans. The findings, published today
in Current Biology, suggest that the industrialized
world’s sleep habits do not differ much from those
that humans evolved to have.
“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,” said Jerome Siegel, leader of the research
team and professor of psychiatry at UCLA’s Semel
Institute of Neuroscience and Human Behavior.
“I feel a lot less insecure about my own sleep
habits after having found the trends we see here,”
added lead author Gandhi Yetish, a Ph.D. candidate
at the University of New Mexico.
The findings do validate some common ideas about
sleep and health, including the benefits of morning
light, a cool bedroom and a consistent wake-up time.
An international authority on sleep, Siegel is a
past president of the Sleep Research Society. For 40
years, he has run a basic sleep research lab in Los
Angeles.
He started studying sleep among traditional peoples
two years ago, asking anthropologists who were
already heading into field to bring along special
watch-sized devices that measure sleeping and waking
times as well as light exposure.
Researchers from Hunter College, Yale University, UC
Santa Barbara and the University of New Mexico
clocked sleep patterns among the Hadza,
hunter-gatherers who live near the Serengeti
National Park, and the Tsimane,
hunter-horticulturalists who live along the Andean
foothills.
Siegel, aided by contacts supplied through a
colleague at Witwatersrand University in South
Africa, gathered measurements among the San
hunter-gatherers in the Kalahari Desert. In addition
to measuring how long and when these adults slept
during the summer and winter, Siegel measured their
body temperatures, the temperature in their
environment and the amount of light to which they
were exposed.
The team, which received support from UCLA, the
National Institutes of Health and the National
Research Foundation of South Africa, collected sleep
records on 94 adults for a total of 1,165 days. The
study is the first on the sleep habits of people who
maintain foraging and traditional hunting lifestyles
in the present day.
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,” said
Siegel, who is also chief of neurobiology research
at the Veteran Affairs of Greater Los Angeles Health
Care System.
Most of the people studied by 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.
“There’s this expectation that we should all be
sleeping eight or nine hours a night and that if you
took away modern technology people would be sleeping
more,” said Yetish, who spent 10 months with the
Tsimane. “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 six hours in the
summer and just under seven 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 said. “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 said.
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,
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 said. “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 said.
For more information
Current Biology
Natural Sleep and Its Seasonal Variations in Three
Pre-industrial Societies
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UCLA - University of California, Los Angeles
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