Sleep-related behavioural adaptations in free-ranging anthropoid primates.

Several aspects of behaviour relating to sleep in monkeys and apes are reviewed, including sleeping site selection, approach to and departure from sleeping sites, social behaviour at the sites, and nocturnal activities. Illustrative examples are given for each topic. Good sleeping sites for primates give protection from predators and/or some physical comfort from the elements and other sources of disturbance. Availability of sleeping sites may determine ranging patterns and whether an area is exploited or not. Times of retiring and resumption of daytime activities are influenced by foraging and ranging requirements. Social relationships and their influencing factors continue during the night, including dominance, kinship, affiliation and sex. Social partners may be used for thermoregulation and for increasing postural stability. Primates show a range of solutions to the problems surrounding sleep, and similarities and differences between monkeys and the large-bodied, nest-building great apes are described. Knowledge of natural sleep-related phenomena in non-human primates can provide valuable insights for human sleep research, and vice-versa.