This week we learned that crocodiles appear to sometimes sleep with one eye open, though that finding, neurologically speaking, needs to be confirmed with an EEG.
The evolutionary trick is called unihemispheric sleep, where, essentially half an animal's brain sleeps while the other half remains awake, with the ability to keep an eye open for anything that looks like trouble.
Crocs aren't the only creatures to exhibit the behavior, however. Some aquatic mammals and birds manage the feat as well. Let's take a look at a sampling of them.
Heads Up: Crocodiles Climb Trees