Sleep, immune defences and parasitism. Interspecific evidence that sleep protects against parasitic infection. (a) The number of white blood cells in peripheral blood increases among species with longer sleep durations. The fitted line is derived from a multiple regression and controls for a significant influence of body mass and activity period. (b) Species that sleep for longer are infected by fewer parasites. Relative infection status takes account of both the number and prevalence of different parasites infecting host species and corrects for differences in sampling effort [29–31].