The Hadza community in Tanzania follows a hunter-gatherer way of life without electric lighting. Although people are often viewed as active mainly by day, recordings from such groups show notable movement after dark rather than complete rest once sunlight fades. This pattern aligns human behavior more closely with certain lemur species than with strictly daytime primates, according to David Samson of the University of Toronto and author of The Sleepless Ape. He describes humans as potentially cathemeral, distributing activity across full 24-hour cycles. The term cathemeral was introduced in 1987 by primatologist Ian Tattersall after observing lemurs in Madagascar that remain active for short periods both day and night. Samson gathered accelerometer data from Hadza participants, who sleep outdoors or in basic shelters, and compared it with measurements from seven lemur species and one gibbon species. The Hadza recorded an average of 52 minutes of activity per hour during daylight and 24 minutes per hour at night. In contrast, the diurnal gibbons showed only two minutes of nighttime activity per hour. These human figures resembled the cathemeral lemurs, which averaged 45 to 48 minutes daytime and 18 to 21 minutes nighttime. A mathematical model of primate evolution indicated that human sleep-wake cycles diverged from those of other primates, possibly due to distinct pressures. Samson suggests humans have adapted through shelter, fire, preparation, lighting, and vigilance, summarized as SHELL, allowing nighttime activity despite internal body clocks. Kathleen Reinhardt of the University of Calgary notes that humans display flexible timing yet remain largely diurnal. She argues that true cathemerality requires sustained behaviors such as feeding or travel in both periods, not merely detected motion. Accelerometer readings alone cannot confirm the purpose of nighttime movement. Separate research on Indigenous Orang Asli communities in Malaysia found sleep times shifted by up to an hour after electricity, new housing, and altered work patterns were introduced.
