They studied their sleeping positions, body movements and sleep patterns, recording the time the primates spent awake and asleep, and whether their sleep was fragmented. They also monitored brain activity, by measuring rapid eye movement (REM), or lighter sleep, versus nonrapid eye movement (NREM), or deeper sleep. In humans, REM sleep is associated with dreaming.
By every measure of sleep quality, orangutans are the ‘better’ sleeper
They confirmed that the orangutans slept for longer, and more deeply than the baboons, suggesting that all great apes do indeed sleep better than monkeys.
“We discovered that by every measure of sleep quality, orangutans are the ‘better’ sleepers; that is, compared to baboons, orangutan sleep is deeper, longer in duration, and less fragmented,” says Dr Samson.
Why they do so is even more revealing.
To date, every population of wild great ape studied builds platforms to sleep on. Gorillas, orangutans, chimps and bonobos create nesting platforms in the trees, whereas modern humans construct beds to lie on.
But every other type of primate does not sleep this way.
Gibbons, the smaller apes, do not construct sleeping platforms, nor do any large monkeys such as baboons. Most monkeys actually sleep sitting in the trees, balancing on a branch, often upright, resting upon their bottoms.
And that difference in sleeping style explains how well they sleep.
Monkeys have more fragmented sleep, fearing predators.
“Sleep quality may be a critical difference between apes and monkeys. Monkeys likely spend more time in ‘light’ sleep due to their less comfortable, less secure, and socially dynamic sleep environments. The trade-off is that they can easily arouse from sleep when a predator is around, or a social partner is active, but the cost is that they don’t achieve the benefits of deep sleep.”
“We apes seem to have innovated an effective way to sleep both securely and comfortably.”
“From an evolutionary perspective, just as the transition from tree branches to sleeping platforms had adaptive benefits, so too did the early hominin transition from sleeping platforms to secure ground sleep."
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