Humans are generally considered diurnal, or mainly active during the day, but this may not be strictly accurate. Data from hunter-gatherers suggests they are fairly active at night, and don’t simply rest and sleep after the sun goes down. 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. Humans sleep the least of all apes – is it the secret to our success? Cathemerality was first described by primatologist Ian Tattersall, who studied the lemurs of Madagascar.
Some lemur species are not primarily active in the day (diurnal) or during the night (nocturnal), but instead are active for a few hours at a time during both periods. Tattersall coined the word “cathemeral” in 1987. To find out if humans are diurnal or cathemeral, Samson compiled data collected from humans and compared it to 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 only active for 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. 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. 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.
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