![]() Sleep rebound is regularly observed in laboratory animals, and suggests importance of the restorative function of sleep, and the detriments an animal can experience should amount of time for sleep be compromised 3, 18, 19, 20, 21, 22. An animal that experiences a stimulus that arouses them from sleep during regular circadian rest patterns would need to reallocate resting time at another portion of the day 5. An important manifestation of sleep homeostasis is the capacity to compensate for the loss of sleep following sleep (or rest) deprivation 3, 18. ![]() The intensity of sleep increases as a function of preceding wake duration, with lowest sleep pressure towards the end of a sleep period 14, 15, 16, 17. Numerous laboratory studies suggest that sleep is homeostatically regulated 13. Despite growing literature on sleep and its functions in the last decades, the need for comparative research in natural environments to further our understanding of sleep ecology, physiology and evolution is becoming increasingly important 2, 5. Another important intrinsic factor, which has a strong influence on sleep amount and intensity, is preceding sleep-wake architecture. This synchronisation is due to retinal photoreceptors and their photosensitivity to light cues, commonly referred to as Zeitgeber 8. The circadian clock provides a rhythmic output to behaviour and physiology, and is synchronised to light levels, allowing animals to anticipate day and night 8, 12. The effect of these extrinsic factors on sleep and activity is mediated by their interaction with endogenous regulatory mechanisms, such as the circadian clock 11. ![]() Such fluctuations include risk of predation, ambient temperature, humidity and light 5, 6, 7, 8, 9, 10. ![]() These variations are superimposed with daily fluctuations in environmental variables that have a strong influence on activity patterns. Even in relatively stable laboratory conditions, the amount and characteristics of sleep and waking vary substantially across 24-h. Birds and mammals can display two different sleep patterns: monophasic sleep, when an animal exhibits a single consolidated bout of sleep in one portion of a day or polyphasic sleep, when an animal displays several short episodes of sleep 2, 4, 5. Sleep is a fundamental requirement for many animals in maintaining cognitive performance and physiological functions 1, 2, 3. Our data suggest that the consolidated monophasic sleep patterns shaped by environmental pressures observed in slow lorises represent phylogenetic inertia in the evolution of sleep patterns in humans. We concluded that well-known phenotypic variability in daily sleep amount and architecture across species may represent an adaptation to changes in the environment. The striking relationship between daily activity patterns, light levels and temperature suggests a major role of the environment in shaping the daily architecture of waking and sleep. The longest consolidated rest episodes were typically clustered near the beginning and towards the end of the light period, and this pattern was inversely related to daily fluctuations of the ambient temperature. All individuals showed a strictly nocturnal pattern of activity and displayed a striking synchronisation of onset and cessation of activity in relation to sunset and sunrise. We fitted seven wild Javan slow lorises ( Nycticebus javanicus) in West Java, Indonesia with accelerometers that collected activity data, and installed climate loggers in each individual’s home range to collect ambient temperature readings (over 321 days in total). Here we report the first sleep study on a nocturnal primate performed in the wild. Only a few primate species have been systematically studied in their natural habitat where environmental variables, including temperature and light, have a major influence on sleep and activity patterns. Among primates, the suborder Haplorhini is considered to have evolved a consolidated monophasic sleep pattern, with diurnal species requiring a shorter sleep duration than nocturnal species.
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