Eye-opening research by zoologists shows crocodiles can sleep with one eye open.
The study suggested that's because only half of a crocodile's brain shuts down at any one time.
Birds, other reptiles and aquatic animals such as dolphins, seals and walruses all have this capability, and it may turn out that land-based mammals, such as humans, are the exception.
The Australian and German researchers who conducted the study said the one-eye approach allowed the open eye to connect to the 'awake' side of the brain, enabling the creatures to respond to threats and prey.
"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," said lead researcher Michael Kelly, of La Trobe University in Melbourne.
"What we think of as 'normal' sleep may be more novel than we think."
Dr John Lesku said humans thought of our own sleep, which involved the whole-brain, as "normal".
"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."