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I investigated sleeping-site preference in habituated wild Japanese macaques (Macaca fuscata) for each season (254 days total) on predator-free Kinkazan Island, northern Japan, during 2000-2007. I focused on the effects of nonpredatory, environmental factors (vegetation type, altitude, and topography), to which little attention has been paid. Macaques used 24-79 sleeping sites in each season (227 sites in total, all on the ground). The frequencies of sleeping sites in each season followed a Poisson distribution, except for spring when several sites were used repeatedly. In spring macaques preferred sleeping in Zoysia japonica grassland, where several staple food species (Berberis thunbergii and Zelkova serrata) are abundant in this season. In summer and fall macaques avoided sleeping in high-altitude forest dominated by Fagus spp., and in the latter season they also preferred Zoysia grassland; these preferences likely reflect an avoidance of strong winds rather than the lower food availability at higher altitudes. In winter macaques avoided sleeping in Zoysia grassland, mainly due to the poor food supply. Macaques preferred valleys to ridges in spring and winter, possibly due to greater densities of shelters such as rocks and fallen trees that facilitate energy conservation in the face of strong/cold winds at night. Additional quantitative data for other mammalian species are needed for generalizations to be made about the importance of nonpredatory factors on sleeping-site preferences.