Ekirch believes it persists, among
Westerners who spontaneously wake in
the middle of the night, “a persistent
echo of a pattern of sleep... dominant
for literally thousands of years.”THE SENTINEL HYPOTHESIS
Within any culture, people also prefer
to rest and rise at different times: In
most populations, individuals range
from night owls to morning larks in
a near bell curve distribution. Where
someone falls along this continuum
often depends on sex (women tend to
rise earlier) and age (young adults tend
to be night owls, while children and
older adults typically go to bed before
the wee hours).
Genes matter, too. Recent studies
have identiied about a dozen genetic
variations that predict sleep habits, some
of which are located in genes known to
inuence circadian rhythms.
While this variation can cause conict
today — between, say, late-rising teens
and early bird parents — it may be the
vestige of a crucial adaptation. Accord-
ing to the sentinel hypothesis, staggered
sleep evolved to ensure that there was
always some portion of a group awake
and able to detect threats.
First proposed in 1966 by psy-
chologist Frederick Snyder, the ideahas gained some recent support. In
a 2017 study in Proceedings of the
Royal Society B, Samson gave activity
trackers, which can approximate sleep
time, to a community of Hadza hunter-
gatherers in Tanzania. Over 20 nights,
there were only 18 one-minute periods
when everyone was asleep. Most of the
time, about 40 percent of the group
was active.
The study “suggested there’s some
type of mechanism set in place where
there’s individuals alert, protecting
everyone, while most everyone is asleep,”
says Samson. This could explain how
our ancestors avoided danger while
enjoying deep, REM-packed sleep.
It also its the idea that the pat-
tern emerged around 2 million years
ago, when ancestors like H. erectus
abandoned the safety of trees for a fully
terrestrial life. Eficient, sentinel-style
sleep may have then spurred advances
in brain power, technology and social
cooperation seen in later hominins.
While logical, the hypothesis must
remain untested. As Samson puts it,
“We’ll never have a Homo erectus to
hook up to an EEG.” DBridget Alex is an anthropologist at
Harvard University and frequent contributor
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A 2017 study of the Hadza, hunter-gatherers of Tanzania, found support for the sentinel
hypothesis: Naturally staggered individual sleep patterns keep the entire group safer.