Intra- and inter-individual variability in the circadian rhythm of body temperature of rats, squirrels, dogs, and horses

In order to explore the often neglected issue of the relationship between intra- and inter-subject variability of circadian rhythms, we evaluated the variability in four parameters of the circadian rhythm of body temperature under controlled laboratory conditions. To avoid the bias of potential selection of an idiosyncratic species, we conducted the study on four different species: the laboratory rat, the thirteen-lined ground squirrel, the domestic dog, and the horse. In rats and squirrels, three of the four parameters of the body temperature rhythm (mean level, amplitude, and regularity of waveform) showed greater inter-subject variability than intra-subject variability. The two variabilities were not different from each other in dogs and horses. Intra- and inter-subject variabilities of acrophase (time of peak) were not significantly different in any of the species, and their magnitudes were similar in all species, which suggests that acrophase is a very dependable parameter in the analysis of circadian rhythms, even though its overall variability is not particularly low.