Since primates spend about half of their life at sleeping sites, knowledge of behavior in the vicinity of sleeping sites and analysis of factors influencing their use is important for understanding the diversity of primates' adaptations to their environment. The present paper reviews recent progress in the ethology and ecology of sleep in diurnal monkeys and apes. Emphasis is given to the following topics: safety from predators at sleeping sites, physical comfort, social behavior, and psychophysiology of sleep. In all cases, study at the group level and at the individual level can provide insights into behavioral adaptations. As well as increasing understanding of behavior in the wild, knowledge of sleep-related behavior can be applied with a view to improving the environment for captive primates. Am. J. Primatol. 46:63–75, 1998. © 1998 Wiley- Liss, Inc.