Variation in mammalian sleep. (a) Mammals exhibit striking differences in both their daily sleep durations and the amount of time they spend in each of the main sleep phases. Each bar denotes a species specific sleeping time, and the shaded portions show the time devoted to NREM sleep (dark blue) and REM sleep (light blue). (b) The phylogenetic relationship between the species in our dataset. This composite tree was assembled from recently published sources [66–70]. The phylogeny was used to generate independent contrasts , and was restricted to the species present in immune investment and infection status analyses. The available data on sleep durations could not always be matched to both haematological and parasite data, thus, some species were only represented in one of the analyses. (c) A selection of species within the dataset, showing the wide-range of sleep durations that have been recorded. Photo credits: B. T. Preston, The Max Planck Institute of Evolutionary Anthropology & Arco Images.