Although mouse lemurs are solitary foragers, they are known to form sleeping associations. I examined several factors that could influence the choice to sleep communally and the composition of sleeping associations in Microcebus griseorufus, an inhabitant of the subarid spiny forest of southern Madagascar. These include the quantity and quality of available sleeping sites, socio-territoriality, predation risk, and thermoregulation. I radiotracked 26 individuals (12 males and 14 females) and recorded 222 uses of 151 sleeping sites. Mouse lemurs slept most often in tree forks and tangles of vegetation and preferred sleeping sites in Alluaudia spp. (36%) and Euphorbia spp. (30%), 2 very common tree genera in the spiny forest. Sleeping associations allowed the animals to use larger and more extensively overlapping home ranges and to have access to more food. Predation risk was much higher during nocturnal activity than during diurnal rest. Sleeping groups were small, usually pairs. Animals showed no signs of vigilance in sleeping sites and did not huddle consistently. Sleeping sites were chosen for their inaccessibility to predators and were well buffered against high and low ambient temperatures. Therefore, sleeping association was better explained by social territoriality than by predation pressure or thermoregulation.