Animals spend a substantial proportion of their lives resting and consequently the choice of a sleeping site can dramatically affect an individual's fitness. Sleeping sites have been implicated in predator defence and thermoregulation and their distribution can affect breeding and social systems in small
mammals. Sleeping sites may constitute a limiting resource for females which experience energetic
constraints to breeding and require shelter for their young. Despite the potential importance of sleeping sites, exhaustive studies of the factors determining their usage patterns are limited to a small number of mammal groups. We investigated how seasonal changes in temperature or the presence of young shape sleeping site usage patterns in a small primate, the grey mouse lemur, Microcebus murinus. Females preferred tree holes over nests and open sites and the usage duration of a particular tree hole site was
affected by the physical characteristics likely to affect its insulation properties. Furthermore, climate variables strongly affected sleeping site choices and nests were chosen more often than tree holes during the wet season. However, when young were present preferences shifted back to tree holes. Preferred tree species used as sleeping sites as well as the properties of tree holes were rare among trees in the study area, suggesting that sleeping sites may indeed be a limiting resource for female grey mouse lemurs. Our results suggest that in the study species sleeping site choices are governed by thermoregulatory considerations and their limited availability could also account for the social grouping.