Cathemeral:
Given the marked variation in abiotic and biotic conditions between day and night, many species specialise their physical activity to being diurnal or nocturnal, and it was long thought that these strategies were commonly fairly fixed and invariant. The term ‘cathemeral’, was coined in 1987, when Tattersall noted activity in a Madagascan primate during the hours of both daylight and darkness. Initially thought to be rare, cathemerality is now known to be a quite widespread form of time partitioning amongst arthropods, fish, birds, and mammals. [“Cathemerality: a key temporal niche,” Daniel T C Cox & Kevin J Gaston, PubMed]
Noted in “Did humans evolve to be active throughout the day and night?” Michael Marshall, NewScientist (29 August 2026, paywall):
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.
