Crocodiles Can Nap With One Eye Open

Crocodiles can sleep with one eye open, a recent study suggests.

Researchers suspect that parts of crocodile brains remain vigilant during sleep.

John Lesku of La Trobe University and his team filmed young saltwater crocodiles in a tank over a 24-hour period and saw that they typically slept with both eyes closed. But in the presence of a human outside the tank, the crocs opened one eye during sleep to check out who the intruder was, according to the findings published in the Journal of Experimental Biology.

“They definitely monitored the human when they were in the room,” Lesku told the BBC. “But even after the human left the room, the animal still kept its open eye … directed towards the location where the human had been—suggesting that they were keeping an eye out for potential threats.”

The team thinks crocs may keep one eye open to protect themselves from potential threats like humans. Baby crocodiles may utilize the same strategy to stay close to their crèche during naps.

Humans and many animals sleep with both eyes closed and their brains fully switched into sleep mode. Some birds and aquatic mammals, such as dolphins, are able to snooze with one eye open, note the authors of the croc study. This is because the brains of those animals remain partially awake during sleep, the researchers say.

The researchers suspect the same brain mechanism of partial wakefulness is behind crocs’ eye-opening maneuver, but more research is needed to confirm that.

The findings may help overturn beliefs about what constitutes normal sleep among living creatures. “The value of the research is that we think of our own sleep as ‘normal’—a behavioral shutdown that is a whole-brain affair,” Lesku says in a statement. But some birds, aquatic mammals and perhaps even some reptiles appear to not sleep the way humans do.

It could be that the way humans sleep is the oddity, he says.