The normal sleep patterns of the spider monkey (Ateles geoffroyi) have not been described yet. The objective of this study was to characterize the electrophysiological patterns, sleeping postures, and sleep‐wake cycle in semi‐restricted spider monkeys. Continuous 24‐hr polysomnographic (PSG) recordings, involving simultaneous recording of non‐invasive electroencephalographic (EEG), electro‐oculographic (EOG), and electromyographic (EMG) activities, were carried out in captive monkeys living in outdoor rainforest enclosures. Electrode placement was done according to the human international 10–20 system. Specific behaviors displayed by monkeys during the sleep‐wake cycles were correlated with the PSG recordings. The nycthemeral distribution of the sleep‐wake cycle was also calculated. The results show that electrophysiological N‐REM sleep patterns in spider monkeys are similar to those observed in other primates, including human beings. Furthermore, a vertical semi‐fetal posture was observed during N‐REM and REM sleep phases. The amount of nocturnal sleep was significantly higher than that of the diurnal period, showing that the spider monkey is a diurnal primate. An outstanding finding was the absence of muscular atonia during the spider monkey's REM sleep, which suggests that arboreal primates have developed a neuromuscular mechanism specialized for sleeping in a vertical posture. Am. J. Primatol. 77:200–210, 2015. © 2014 Wiley Periodicals, Inc.