Fig 1. Circadian organisation is not ubiquitous in mammals.

Activity patterning in (from the top) humans, mice (Mus musculus), voles (M. arvalis), and reindeer (R. tarandus) under 24-h LCs and NLCs. All four species display pronounced 24-h rhythms of activity under LC. These rhythms persist under NLC in humans and mice but not in voles and reindeer. Data for humans are from bunker experiments in which subjects were initially exposed to changes in light intensity synchronised to the solar day (LC) and then allowed to free-run with only self-imposed changes in light level (NLC [6]). For mice and voles, experimental light and dark phases are represented by horizontal white and brown bars, respectively. For the reindeer, free-living in their natural environment, natural photoperiod (onset and offset of civil twilight) is indicated by vertical yellow lines on the first day of each actogram, and the NLC regime was the polar night at 78° north latitude. Data for one individual of each species under each light regime are presented as double-plotted actograms. Black bars represent activity. LC, light–dark cycle; NLC, no-light cycle. Redrawn from [6–9].