Did humans evolve to be active throughout the day and night?

Recent research has challenged the notion that humans are strictly diurnal, or primarily active during the day. Data from hunter-gatherer communities suggests that humans may be more active at night than previously thought. This has led scientists to suggest that humans may be "cathemeral," meaning they spread their activities throughout each 24-hour period, rather than being primarily active during the day and resting at night.

David Samson, a researcher at the University of Toronto, compares the human sleep pattern to that of some lemur species. Instead of being primarily diurnal or nocturnal, humans may be more like cathemeral lemurs, with activity spread across the day and night. The study compared activity patterns of humans to those of seven species of lemur and one species of gibbon, using wrist-mounted accelerometers.

Humans were found to be more active than the strongly diurnal gibbons, but closer to the cathemeral lemurs, with activity levels of 52 minutes per hour during the day and 24 minutes per hour during the night.

Samson also built a mathematical model of primate evolution, finding that humans do not follow the same optimal pattern of sleep and activity as other primates. This suggests that human sleep-wake activity may have evolved under different selective pressures. The researchers believe that our ability to use shelter, hearth, environmental preparation, light, and lookouts (SH-SHELL) has allowed us to remain active at night, buffering us from some of the ecological costs of being active or asleep at unusual times. However, they emphasize that this does not mean we have completely erased our biology.

While some researchers, like Kathleen Reinhardt at the University of Calgary, still believe humans are primarily diurnal, they acknowledge that our sleep patterns are more flexible than previously thought. The definition of cathemeral activity is stricter than simply evidence of activity at night, requiring meaningful behaviors such as feeding and traveling.

The accelerometer data alone may not be enough to determine the nature of nighttime activity. However, it is clear that humans are not tied to strict sleep rhythms. Changing sleep patterns among Indigenous Orang Asli people in Peninsular Malaysia, who have recently gained access to permanent electricity, further support the idea of adaptable human activity patterns.