Samson and his team observed that Hadza sleep patterns were rarely in sync. Out of over 220 hours of observation, there were only 18 minutes when all adults were sound asleep simultaneously. On average, more than a third of the group was alert, or dozing very lightly, at any given time.
Usually the Hadza participants went to bed shortly after 10 pm and awoke at around 7 am, but some retired as early as 8 pm and woke up by 6 am. Still others stayed up past 11 pm and slept until after 8 am. Daytime naps were frequent.
Conditions while the Hadza slept were family-oriented, to say the least.
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“The Hadza do not sleep on raised cots, but simple ground beds that consist of a textile or an animal hide,” Samson said. “Couples and families consistently sleep on the same bed, and the average bed has 2.4 individuals on it, but I measured up to seven on one, including a nursing mom.”
“This proximity, I am sure, leads to a type of shared ‘asynchrony,’ where when one individual wakes up within a hut, it may increase the likelihood of another waking up,” he added.
Several factors are behind the varying sleep habits. As previously indicated, the number of people in a given sleep group matters, as does the timing of snooze phases, such as REM (rapid eye movement) sleep, a deeper rest during which dreaming tends to occur.
The primary driver, however, is each individual’s chronotype.
“Chronotype is defined as the individual propensity for sleep and activity at particular times during a 24-hour period,” Samson explained. “It's also moderately heritable and varies through the lifespan — thus it is controlled by genes, environment, and age.”
As a result, some humans wind up being morning people and others become night owls. Chronotype variation is found not just in primates but also in other animals.