New Scientist
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Humans are usually classified as daytime animals, but research on the Hadza hunter-gatherers suggests our ancestors may have been active more regularly at night than assumed.
Did humans really evolve to be active only in the day?
Author: Michael Marshall
HUMANS are generally considered diurnal, or mainly active during the day, but data from hunter-gatherers suggests they are fairly active at night.
This finding means that the human sleep pattern is closer to that of some lemur species, says David Samson at the University of Toronto in Canada, author of The Sleepless Ape. Instead of being primarily active in the day and restful at night, humans may be “cathemeral”, meaning they spread their activities throughout each 24-hour period. “The key point is that day and night activity are both regular parts of the behavioural repertoire,” says Samson.
To find out if humans are diurnal or cathemeral, Samson compiled data collected from humans and compared it with recordings from non-human primates: seven species of lemur and one species of gibbon. In all cases, activity was recorded using wrist-mounted accelerometers. Because many humans now sleep in permanent houses and have access to electricity, Samson used accelerometer recordings from Hadza people: hunter-gatherers living in Tanzania who generally sleep in the open or in shelters made from branches and grass, without artificial light.
The Hadza were active for an average of 52 minutes per hour during the day and 24 minutes per hour during the night. This contrasted with the strongly diurnal gibbons, who were active for only 2 minutes per hour at night. The humans were closer to the cathemeral lemurs, who were active for 45 to 48 minutes per hour in daytime and 18 to 21 minutes per hour at night (Journal of Human Evolution, doi.org/rkn6).
Samson also built a mathematical model of primate evolution, to see whether all primates were evolving towards the same optimal pattern of sleep and activity. He found that humans didn’t match other primates.
Flexible rhythms
“Human sleep-wake activity may have evolved under somewhat different selective pressures than in other primates,” he says.
Samson argues that humans have evolved to defy our internal circadian rhythms. While our body clocks encourage us to be active in the day and asleep at night, our societies and technologies allow us to do otherwise. By using shelter, hearth, environmental preparation, light and lookouts – which Samson groups under the acronym SHELL – we can remain active at night, he says. “That SHELL did not erase our biology. It buffered us from some of the ecological costs of being active or asleep at unusual times.”
“Humans show very flexible patterns in when we sleep and when we’re active,” says Kathleen Reinhardt at the University of Calgary in Canada, but she thinks we are still primarily diurnal.
The definition of cathemeral is stricter than simply evidence of activity at night, she says. “We’re talking about more meaningful behaviours such as feeding and travelling, and those kinds of behaviours occurring in both periods, not just simply detecting movement in the light and the dark,” she says. The accelerometer data isn’t enough to determine the nature of nighttime activity, she says.
Credits: TCA, LLC.