Whether you’re a human or a great ape, chances are you get lots of sleep. But how much and where do monkeys sleep? That’s what Charles Nunn and David Samson at Duke University in Durham, NC, and the University of Toronto Mississauga, ON, set out to find out by comparing sleeping patterns among 30 different species.
The authors used implanted telemetry to record EEG and ECoG in unrestrained male rhesus monkeys (Macaca mulatta) housed individually under a 16:8 light-dark cycle. Sleeping periods lasted about half of the dark period and were relatively consolidated. During these sleep periods, the proportion of time spent in each sleep stage varied significantly from animal to animal (Table 4). In accord with predation avoidance, Bale monkeys entered their sleeping trees shortly before sunset and exited them at sunrise. They also reused their sleeping sites, a strategy that may enhance their familiarity with the area and therefore improve their chance of escaping attacks by nocturnal predators such as spotted hyenas. Furthermore, they selected sleeping trees with dense foliage above and below them, which would conceal them from predators and offer shelter from the cold weather.
The authors found that NREM delta activity peaked at about the third hour of sleep and decreased monotonically thereafter, a pattern observed in all animals. Moreover, the frequency of REM sleep was proportional to the duration of NREM delta activity. The authors suggest that these results support the hypothesis that NREM delta activity is important for learning, memory and cognition.