Anybody who has ever struggled to get enough sleep knows just how much in life can interfere with our rest, and just how detrimental this can be to our health and happiness. Researchers from the Max Planck Institute of Animal Behavior (MPI-AB) and the University of Konstanz in Germany, in collaboration with scientists at the Universitas Nasional in Indonesia, have found that some of our closest living relatives, orangutans, face similar issues, and have a very familiar coping strategy: napping.
“Moving through the canopy, finding food, solving problems, navigating social relationships; these are all tiring and cognitively demanding tasks,” says Alison Ashbury, 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.”
Tracking sleep in the treetops
The research team worked in the Indonesian rainforest to examine the sleep patterns of wild adult orangutans, which have never been studied specifically for how they solve challenges of sleep. Doing so opened a window into understanding how sleep evolved in great apes and our own human ancestors. The scientists collected data from 53 adult orangutans over 14 years at the Suaq Balimbing Monitoring Station in Sumatra, recording in total 455 days and nights of orangutan behavior.
But tracking sleep in the wild provided logistical challenges for human observers. Much like us, wild orangutans sleep in beds, known as “nests”, which provide a safe place for resting. Every night, an adult orangutan will settle in a spot high up in the rainforest canopy. There, it will spend about ten minutes building a nest: bending, breaking, and weaving together tree branches to create a solid platform, complete with a leafy mattress and pillow for comfort. Mothers share nests with their nursing young, but otherwise, with very few exceptions, adult orangutans sleep alone. At dawn, they leave their nests to start the day.
“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 and a group leader at MPI-AB. “Eventually, everything goes quiet and still. And the reverse happens in the morning.”
It was that silent stretch in the middle that researchers called the “sleep period” and that they used as an indicator of sleep. They found that orangutans’ sleep periods were, on average, nearly 13 hours long. In the paper, the team cited past work in both captive orangutans and wild baboons that showed strong correlations between sleep period and actual time spent sleeping. This suggests that, although the team couldn’t directly measure sleep itself among these wild orangutans, the sleep period that they could measure is a robust indicator of actual sleep.
The researchers also found that several factors were associated with shorter overnight sleep periods: going to sleep near other orangutans, colder nighttime temperatures, and further daily travel.
“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, a scientist at MPI-AB and the University of Konstanz. “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. They’re prioritizing being social over sleeping, or their sleep is being disrupted by others nearby, or even both.”
The power of the power nap
To understand how orangutans recover from lost sleep, the team analyzed how the duration of nap periods changed in relation to the previous night’s rest. They found a clear compensatory effect: orangutans’ nap periods were longer on days after they had shorter night time sleep periods, and when they did nap, they napped 5 to 10 minutes longer for every hour less sleep the night before.
“For people, even a short nap can have significant restorative effects,” says co-author Meg Crofoot, director at MPI-AB and a professor at the University of Konstanz. “It’s possible that these naps are helping orangutans reset physiologically and cognitively after a poor night’s sleep, just like they do in humans.”
Day nests are central to this strategy. Compared to orangutans in many other populations, the Suaq orangutans more frequently build nests during the day. These nests are simpler and faster to build than night nests, typically taking under two minutes, but still offer a stable and secure place to nap.
“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 these findings may also relate to orangutan cognition. The Suaq population is known for its tool use and cultural complexity—traits that may require robust mechanisms to buffer against sleep deprivation.
“Among all studied orangutan populations, the Suaq orangutans arguably exhibit the widest range of cognitively demanding behaviors,” says Schuppli who is the research director at the Suaq research site. “This may be linked to their relatively high propensity for daytime nest use. Either they need these high-quality naps to meet their cognitive demands, or their cognitive abilities can come about because they take high-quality naps in day nests so often.”
This napping strategy may also be made possible by their semi-solitary lifestyle. While primates in cohesive groups must constantly coordinate with others, orangutans are free to nap when and where they choose. On 41% of observed days, orangutans took at least one nap, and they averaged a total nap period of 76 minutes.
Studying sleep in the wild
This study contributes to a growing body of evidence that wild animals must trade-off between their need for sleep and their need to fulfil other social and ecological demands. While the neural and physiological processes and the benefits of sleep are well-studied in laboratory settings, Crofoot, who is leading an ERC-funded project on sleep in the wild, points out: “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. Why did animals, from humans to primates to spiders to jellyfish, evolve to spend such large portions of their lives in this vulnerable unconscious state? 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.”
