Early humans may have evolved as cathemeral, not strictly diurnal
Research into hunter-gatherer societies suggests ancestral humans used flexible sleep-wake cycles to survive.
Humans may not have evolved as strictly diurnal creatures. Recent research into ancestral behaviors suggests that early humans likely followed a 'cathemeral' activity pattern, distributing bouts of wakefulness and sleep across both day and night rather than adhering to a rigid daylight schedule.
Evidence from contemporary hunter-gatherer groups, such as the Hadza, indicates that humans may be cathemeral, spreading their activities throughout the full 24-hour period. Unlike diurnal organisms, which are active only during the day, or nocturnal ones, which operate at night, cathemeral organisms are active at irregular intervals regardless of the light phase. This flexible approach to wakefulness allowed early humans to forage and survive more effectively by adapting to immediate environmental needs.
The Primate Connection
This biological flexibility is not unique to humans but is a known trait among other primates. Certain lemur species, most notably the ring-tailed lemur, serve as classic examples of cathemeral behavior. These primates shift their activity levels based on seasonal changes, food availability, and ambient temperature. By mirroring this pattern, early humans could have optimized their energy expenditure and resource gathering, moving away from the strict, consolidated nocturnal sleep block that characterizes much of modern society.
Redefining Sleep Dysfunction
If humans are evolutionarily cathemeral, the modern struggle with insomnia or 'night owl' tendencies may not be a biological dysfunction of the circadian rhythm. Instead, these patterns could be vestigial traits—remnants of an ancestral survival strategy that prioritized flexibility over a synchronized clock. The rigid expectation of a single, consolidated block of nighttime sleep may be a cultural imposition rather than a biological necessity.
Future Implications
Understanding this evolutionary flexibility could fundamentally reshape how the medical community approaches sleep hygiene and the management of shift work. It also provides a new lens through which to view the biological impact of artificial light pollution, which may be disrupting a naturally fluid system rather than a strictly diurnal one. While the core theory of human cathemerality is supported by observations of the Hadza, further research is needed to determine the exact evolutionary triggers that shifted most modern populations toward a more synchronized diurnal cycle.