In the wilds of Africa, chimpanzees consistently choose to make their sleeping nests in a particular tree that offers the "just right" kind of comfort that Goldilocks famously preferred.
That's according to a new study in the journal PLOS ONE that could also bolster a theory that solid shut-eye may have been a key to human evolution.
In the latest study, scientists measured the "stiffness and bending strength" of seven trees most commonly used by chimps to make their sleeping nests in Uganda's Toro-Semliki Wildlife Reserve. The scientists then looked at hundreds of nests.
National Geographic writes: "Of the 1,844 chimpanzee nests studied, 73.6 percent were made from a sturdy tree called Ugandan ironwood — even though that species made up only 9.6 percent of trees in a survey of the region.
"Despite the fact it's relatively rare, they're saying 7 out of 10 times, 'I want to sleep in this species,' " study leader David Samson was quoted by NatGeo as saying.
In the PLOS ONE abstract, the authors said it appears the chimps preferred "a compliant yet constraining structure [reducing] stress on tissues."
"[The] functional concavity of the nests obviates the need to adjust posture during sleep to prevent falls," the authors added.
A sleep quality hypothesis that holds "that apes construct sleeping platforms to allow uninterrupted sleep and to promote longer individual sleep stages" seems to be supported by the findings, scientists say.
So, what do snoozing chimpanzees have to do with our own evolution?
National Geographic says:
"Sometime in the Miocene period, 23 to 5 million years ago, ancient apes changed their sleeping locations from branches to platforms. That, in turn, led to a better night's sleep.
"Studies in both humans and orangutans show that better quality sleep, with longer periods of rapid eye movement, improves cognition and memory. Ancient apes' improved slumber, then, may have led to the development of bigger brains.
"But it's also possible that apes' big brains may have led to the need for more sleep, not the other way around, noted [biological anthropologist Aaron] Sandel (who is not involved in the chimp bed study).
"In any case, Samson said, an added boost in cognition certainly gave apes and humans an evolutionary edge.
" 'Big brains,' he said, 'need big pillows.' "