Watching an orangutan sleep is like watching a giant, orange baby slumbering sweetly.
These huge great apes like to get into bed, and nestle down for a long and deep night’s sleep, their eyes occasionally dancing behind their eyelids, perhaps dreaming a fleeting orangutan’s dream.
Watching a baboon sleep is more like watching a small bitter paranoid person desperately trying to get some shut eye.
They sleep badly; sitting upright, balancing on their bottoms, minds whirring, constantly fearful that something or someone is after them.
Which begs an important question: why does an orangutan sleep so soundly, whereas its primate relative, the baboon, suffers a fretful night’s rest?
The answers, scientists are learning, are rooted deep in our evolutionary history. They help explain, in part, how great apes including humans were able to evolve into the beings we are today, and also why humans routinely prefer to sleep in beds.
“Sleep has only recently been acknowledged as a potentially critical factor in human evolution,” says anthropologist David Samson of Duke University in Durham, North Carolina, in the US.
Yet scientists have rarely studied how sleep may have affected our development as a species.
So Dr Samson and colleague Robert Shumaker of Indiana University in Bloomington, in the US, decided to do just that.
They chose two primate species to study, publishing the results in the American Journal of Physical Anthropology.
“We chose orangutans because they were a species of ape that had yet to be studied by sleep scientists,” Dr Samson told BBC Earth.
“Furthermore, they are the most distantly related great ape to humans, and therefore are an important species to generate comparative data.”
“The baboons were chosen specifically due to their being one of the largest bodied monkeys that do not use sleeping platforms. Essentially, I wanted to control for body size and ask the question: why do both these large-bodied primates differ so remarkably in their sleep behaviours?”
The scientists videoed five orangutans and 12 baboons sleeping in captivity over periods of one to four months.
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.
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.
This difference in sleeping style may have played a fundamental role in the evolution of great apes, including humans.
And sleeping in beds confers another significant evolutionary advantage, it allows apes to sleep more deeply, getting more deep, nonrapid eye movement (NREM), sleep.
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