The rumor is true. Crocodiles, in fact, do sleep with one eye open.

The new research, published this week in the Journal of Experimental Biology, suggests crocodiles utilize unihemispheric sleep, resting just half their brain at a time.

Crocs would likely prefer to get more complete shuteye -- with two shut eyes -- but it's a dangerous, unpredictable world out there. Scientists at Australia's La Trobe University say an open eye allows half-asleep crocodiles to respond to threats, passing prey and the needs of young crocodiles.

Some species of birds and aquatic mammals, including dolphins, have been observed sleeping with one eye open. But this is the first study to detail the behavior among crocodiles.

In observing crocodiles, both in captivity and the wild, researchers found that the reptiles were more likely to keep one eye open when humans were present -- and that one wakeful eye was always trained on the human or humans.

"These findings are really exciting as they are the first of their kind involving crocodilians and may change the way we consider the evolution of sleep," lead researcher Michael Kelly said in a press release. "What we think of as 'normal' sleep may be more novel than we think."

Humans tend to think of their behaviors and physiological tendencies as normal. But whole brain sleep may actually be an anomaly, researchers say.

The more scientists understand how other animals sleep, the better they can fit human sleep into the broader picture.

"The value of the research is that we think of our own sleep as 'normal' -- a behavioral shutdown that is a whole-brain affair," said John Lesku.

"And yet, some birds and aquatic mammals sleep unihemispherically with one eye open," Lesku continued. "If ultimately crocodilians and other reptiles that have been observed with only one eye closed are likewise sleeping unihemispherically then our whole-brain (or bihemispheric) sleep becomes the evolutionary oddity."