Orangutans have been found to nap during the day to make up for lost sleep, according to a new study in the Indonesian rainforest.
The research reveals how these great apes balance rest with the demands of daily life – and it seems they behave in a way very similar to humans.
“Moving through the canopy, finding food, solving problems, navigating social relationships; these are all tiring and cognitively demanding tasks,” says Alison Ashbury, a scientist at the Max Planck Institute of Animal Behavior (MPI-AB) and the University of Konstanz, and the study’s first author.
“When an orangutan doesn’t get enough sleep, it does what any sleep-deprived human might do: it climbs into bed, lies down, and takes a nap.”
The findings, published in Current Biology, shed light on how one of our closest living relatives manages sleep in a complex, natural environment.
Studying sleep
The team studied data on 53 adult orangutans and recorded 455 days and nights of sleep behaviour at the Suaq Balimbing Monitoring Station in Sumatra. They found that orangutans typically enjoy a sleep period of nearly 13 hours overnight, snoozing high in the rainforest canopy in specially built tree nests.
These 'night nests' are constructed fresh each evening using branches and leaves, shaped into a stable platform with a leafy base and even a pillow. Mothers will share these nests with their young, but adult orangutans almost always sleep alone. From the forest floor, researchers could only hear movements and rustling, but this stillness proved to be a reliable indicator of sleep.
“From our point of view on the ground, we usually can’t see orangutans at all in their night nests, but we can hear them rustling around, getting comfortable,” says Caroline Schuppli, the study’s senior author. “Eventually, everything goes quiet and still. And the reverse happens in the morning.”
Several factors were linked to shorter night-time sleep periods: cold weather, travelling longer distances during the day, and proximity to other orangutans. The latter was particularly striking, suggesting that even orangutans experience disrupted sleep from social disturbances.
“We thought it was really interesting that just being near other orangutans when building a night nest was linked to shorter sleep periods,” says Ashbury. “Imagine you stay up late hanging out with your friends, or your roommate is snoring so loudly in the morning that you get up early. I think it’s a bit like that.”
Orangutans take power naps
To make up for this lost sleep, the orangutans were observed taking longer naps during the day. In fact, for every hour of reduced sleep at night, they added around 5 to 10 minutes to their nap time the next day. On 41% of observed days, orangutans napped at least once, with nap periods averaging 76 minutes.
These naps usually took place in simpler, quickly made 'day nests'. Built in under two minutes, they were less elaborate than night nests but still provided enough comfort for a proper rest.
“Day nests are less sophisticated, have fewer comfort elements, and are made quicker than night nests,” says Schuppli. “But even so, when we’re able to see an orangutan resting in a day nest, we see that their bodies are relaxed and their eyes are closed. They really do appear to be sleeping.”
The researchers believe this habit may support the orangutans’ impressive cognitive abilities. The Suaq population is known for its use of tools and social learning, traits that may require more restorative rest. Their semi-solitary lifestyle also allows them the freedom to nap when needed, without having to co-ordinate with a group.
“Among all studied orangutan populations, the Suaq orangutans arguably exhibit the widest range of cognitively demanding behaviours,” says Schuppli. “This may be linked to their relatively high propensity for daytime nest use.”
The study highlights the value of observing animals in their natural habitats. “Studying sleep in the wild, in the natural social and ecological conditions under which it evolved, is important to broadening our understanding of the evolutionary origins and the ultimate functions of sleep,” says Meg Crofoot, co-author of the study.
“If we’re going to answer this question, we need to bring sleep research out of the lab and into the field. Studies such as this one contribute to that effort.”
Top image: Cissy, a Sumatran orangutan mother taking a nap in her day nest. Credit: Natasha Bartalotta / Suaq
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