Humans have long been considered daytime animals, but new research suggests our natural activity pattern extends much further into the night than expected.
Humans have always been filed under diurnal: awake by day, asleep by night, like most primates.
But new research suggests that label may be slightly wrong. People, it turns out, may be far more active after dark than anyone assumed, closer in pattern to certain lemurs than to daytime-bound primates like gibbons.
The work was conducted by David Samson from the University of Toronto.
Samson suggests that humans may fit a category called “cathemeral,” meaning activity gets spread across both day and night rather than concentrated in daylight hours alone.
Both day and night activity appear to be regular, built-in parts of human behavior, not an occasional deviation from a daytime norm.
A word borrowed from Madagascar’s lemurs
The term itself isn’t new. Primatologist Ian Tattersall coined “cathemeral” back in 1987 while studying lemurs in Madagascar.
Some lemur species, he found, don’t commit fully to being active by day or by night. Instead, they operate in bursts, a few hours here, a few hours there, scattered across the full 24-hour cycle.
Samson borrowed that framework and applied it to humans for the first time in a systematic way.
Testing the idea against real data
To find out where humans actually fall on that spectrum, Samson pulled together activity data from people and compared it against recordings from eight non-human primates: seven lemur species and one gibbon species.
Everyone in the comparison wore wrist-mounted accelerometers, devices that track movement over time.
There was a catch with the human data, though. Most people today sleep indoors, with electric lighting readily available, conditions that don’t reflect much of human evolutionary history.
So Samson turned to accelerometer recordings from the Hadza, a hunter-gatherer community in Tanzania who typically sleep outdoors or in simple shelters built from branches and grass, without artificial light.
What the numbers showed
The results placed humans well outside the activity pattern seen in the strictly diurnal gibbons.
The Hadza were active for an average of 52 minutes out of every daytime hour, and still active for 24 minutes out of every nighttime hour.
Compare that to gibbons, who moved for just 2 minutes per hour after dark. The Hadza numbers landed much closer to the cathemeral lemurs in the study, who averaged 45 to 48 active minutes per daytime hour and 18 to 21 per nighttime hour.
Samson also built a mathematical model to test whether all primates were converging on the same evolutionary sleep pattern over time.
Humans didn’t fit that trend. He concluded that human sleep-wake activity likely evolved under different pressures than it did in other primates.
Fighting our own circadian clock
If our internal circadian rhythm still pushes us toward daytime activity and nighttime rest, why does actual behavior look so different?
Samson’s answer involves technology and social organization. He points to a set of tools he groups under the acronym SHELL – shelter, hearth, environmental preparation, light, and lookouts – all of which let humans stay active well outside the hours our biology naturally favors.
That toolkit didn’t rewrite our underlying biology but it did shield us from many of the ecological costs that would normally come with being awake or asleep at unusual times.
Not everyone is convinced
Not all researchers are ready to fully reclassify humans as cathemeral. Kathleen Reinhardt at the University of Calgary agrees that human sleep patterns are remarkably flexible, but she still considers people primarily diurnal at their core.
Reinhardt argues the definition of cathemeral demands more than simple movement data.
It requires evidence of meaningful behavior, feeding, traveling, and similar activities, occurring in both day and night – not just any detectable motion picked up by an accelerometer.
There’s a real difference between getting up briefly for a glass of water and getting up to work for an hour or going cycling.
Accelerometer data alone, she added, can’t distinguish between the two.
Sleep that bends with circumstance
Still, Reinhardt agrees on one core point: humans clearly aren’t locked into one rigid sleep rhythm. Mounting evidence points to people adjusting their activity patterns as their environments shift.
In a study published earlier this month, Reinhardt and colleagues documented exactly that kind of shift among Indigenous Orang Asli communities in Peninsular Malaysia, some of whom recently gained access to permanent electricity.
Reinhardt described finding highly adaptable sleep patterns following the introduction of electricity, changes in housing, and new types of labor, all within a fairly short timeframe.
These are communities undergoing rapid lifestyle changes right now and sleep timing can shift by as much as an hour later once power lines arrive.
Whether humans are truly cathemeral or simply diurnal with a lot of built-in flexibility may still be up for debate.
But both researchers agree on the bigger picture: the rigid line between “day person” and “night person” was probably never as fixed as it looked.
The study is published in the Journal of Human Evolution.
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