A sleep duration in the six main groups of endothermic animals depending on the geological time of appearance of the group in evolution. Regression lines and statistics in the figure were calculated using the OLS method. Using the weighted least squares (WLS) method, p = 0.01548 B activity duration (24 minus sleep duration) as a function of geological time since taxon’s divergence, in mya, in different groups of endotherms. Regression lines and statistics in the figure were calculated using the OLS method. Using the weighted least squares (WLS) method, p = 0.01548. The total duration of sleep is an indicator of the reverse value of total activities.