It’s difficult to know how dinosaurs slept because most dinosaurs didn’t die that way.
Finding a dinosaur in a sleeping position is uncommon because most fossils we’ve found show dinosaurs in a classic death pose with their bodies contorted, fearing their imminent demise.
The only way we can know how a dinosaur slept would be if they fossilized in a sleeping position, and while this has happened, it’s rare. According to Scott Persons, a paleontologist at the College of Charleston in Charleston, South Carolina, a few key specimens found in recent years have provided some clues into the sleeping world of these ancient creatures.
Just like many modern animals, dinosaurs might have sought out comfortable, safe positions for sleeping. It’s hard to know how dinosaurs slept on the whole because there were so many different varieties, but we do know key details about certain species.
“In a few specific cases, we know a great deal about how dinosaurs would have slept because we found the skeletons of sleeping dinosaurs,” says Persons.
These specimens, he says, are reminiscent of Pompeii in that they were buried very quickly by volcanic ash, which preserved them for millions of years in a peaceful sleeping pose.
A recent paper, published in the November 15, 2023 issue of the journal PLOS ONE, documents an Alvarezsaurid dinosaur found in Mongolia in a sleeping position. Alvarezsauridae was a small family of dinosaurs about the size of a flightless bird. The specimen was found in a sleeping position, all curled up with its neck and tail wrapped around its body.
(Credit:Kohta Kubo,Yoshitsugu Kobayashi, Tsogtbaatar Chinzorig, Khishigjav Tsogtbaatar)
Holotype of Jaculinykus yaruui gen. et sp. nov. (MPC-D 100/209).
(A) Photograph of the specimen. (B) Explanatory drawing of (A). Highlighted areas refer to the indication of the skeletal elements; skull in green, tail in grey, pectoral girdle and forelimbs in red, pelvis and hind limbs in purple. (C) Reconstruction of Jaculinykus yaruui gen. et sp. nov. Grey areas are missing parts.
“The fossil was hunkered down, with legs and arms folded,” says Persons. “One of the most adorable fossils that you ever did see.”
Another sleeping dinosaur fossil was found in China curled similarly up in an underground burrow. Two 125-million-year-old perfectly preserved Ornithopod specimens were found in the Lujiatun Beds in Northeast China, according to a study published in the September 2020 issue of PeerJ Scientific Journal.
Animals that tend to sleep curled up are likely concerned with holding in their body heat. This means they’re less likely to sleep spread out like crocodiles or lizards, who both get their heat from external sources. These animals burn ample energy producing heat so they would want to keep it tightly held within their bodies while they slept. This was true of both the Alvarezsauridae and the Ornithopod specimens.
There’s still so much we don’t know about sleeping dinosaurs. For example, how did the huge, long-necked sauropods sleep? Did they sleep curled up lying down, or did they sleep standing up for protection and because of blood flow?
“Modern giraffes, for example, sleep standing up with their necks still fully extended because if they slept curled up, their powerful hearts would send too much blood to their heads and cause them to black out,” says Persons.
Additionally, there’s no research to prove this, but, says Persons, it’s been suggested that the large pubic bone in Tyrannosaurus was “basically a kickstand” to hold it up while the monstrous predator was snoozing.
The question of whether dinosaurs were nocturnal (active at night) or diurnal (active during the day) is complex, as dinosaurs were a highly diverse group of animals.
We know a few dinosaurs that have disproportionately large eyes compared to their body size, including Velociraptor, the chicken-sized predator that lived in Asia during the Late Cretaceous, and Troodontidid, a small theropod that lived from the middle Jurassic to the late Cretaceous. This tells us that they could have been nocturnal animals that did most of their sleeping during the day.
Still, it would be difficult for us to know beyond a few species which types of dinosaurs slept during the day and which of them slept at night. We also don’t know how the massive flying and marine reptiles that lived during the age of dinosaurs slept because we haven’t found any specimens that show them in a snoozing state.
The animals of today all sleep in different ways depending on the species, and this was likely true of dinosaurs — some curled up, some burrowed, some slept standing, and others spread out. The sleeping rituals of these prehistoric creatures still largely remain a mystery, with the exception of a few that saw their demise as they slept.