A gibbon wearing a wrist accelerometer moves for about two minutes in an average nighttime hour. Two minutes. The rest of the dark period is close to flat, a stretch of near-total stillness running from dusk to dawn before it breaks open into daytime motion. That is what textbook diurnality looks like when you sample it minute by minute.
A Hadza forager, wearing the same model of device, logs 24 minutes of movement in an average nighttime hour.
That gap is the core of a paper by David Samson of the University of Toronto Mississauga, published in the Journal of Human Evolution,1 which asks whether the standard classification of Homo sapiens as a diurnal primate survives contact with field data. His answer is a qualified no, and the qualification matters as much as the no.
Samson calls the idea the Human Diel Flexibility hypothesis. “Diel” just means the full 24-hour cycle, day plus night. The proposal is that humans retain thoroughly diurnal circadian physiology, the internal clock documented since Jürgen Aschoff’s isolation experiments in the 1960s, while expressing an activity pattern much less day-bound than that physiology would predict. Aschoff concluded from those experiments that humans belong to the light-active organisms. Samson is not disputing the clock. He is disputing what we do with it.
The comparison runs on actigraphy, and its strength is that all of it came off the same instrument. The dataset pulls together three sources: seven lemur species recorded at the Duke Lemur Center, two gibbon species (Hylobates pileatus and Hylobates moloch) at the Gibbon Conservation Center in Santa Clarita, and 33 Hadza adults recorded in bush camps near Lake Eyasi in northern Tanzania during 2016. Everyone wore a MotionWatch 8, sampling motion at 50 Hz and binning it into minute-by-minute counts. Ten species, six genera, 1,227 individual 24-hour periods, 193 individuals. The Hadza were the obvious human sample precisely because they sleep in the open or under grass and branch shelters without electric light, which removes the biggest confound in any comparison of this kind.
The summary statistic is a ratio Samson calls the DNTR, day-to-night time ratio: mean active minutes per daytime hour divided by mean active minutes per nighttime hour. A value of 1 means equal measured activity in both periods. Higher values mean a day-biased animal.
Gibbons came in at 35.60. The lemur genera clustered between 2.42 and 2.93. Humans landed at 2.36, the lowest value in the sample.
Two things about that number deserve care before anyone runs with it. First, humans were not merely more active at night; they were the most active genus overall, logging 52.31 active minutes per daytime hour against the gibbons’ 35.56. Part of what the ratio captures is simply that foragers move a lot. Second, epochs falling within civil twilight were coded as day, so the human nighttime figure is not dusk activity smuggled in under a different name. It is movement in the dark.
The captivity problem cuts the other way and Samson flags it himself. The lemurs and gibbons were living under husbandry schedules, with feeding times, enrichment, visitors, and ambient lighting shaping when they moved. The Hadza were living their lives. Comparing a wild forager to a captive gibbon and concluding something about the human lineage requires holding the result loosely, which is why the paper frames study 1 as an evidence-of-similarity test rather than a reclassification.
