Dolphins sleep with one eye open.

Yes, Dolphins sleep with one eye open, that skill is necessary for survival. At first I would like to inform that Dolphins don’t sleep in one long chunk like we do. They take only 15 minutes to 20 minutes naps throughout the day and night. But the bigger factor is that, when dolphins sleep, only half of their brains get shut down, which enables them to remain vigilant to the threat of life from predators and to regulate their breathing to avoid drowning.

Even Scientists have discovered that instead of falling in sleep, one of dolphin’s hemispheres of its brain is active. "Its mean, when one hemisphere of Dolphins is resting, the other hemisphere is active. so that the dolphin can be sure to open its blowhole above the water."

Whichever brain hemisphere is active, the eye on  opposite side will remain open always. This is good for Dolphins because they are able to perform both swimming to the surface to breathe and for keeping a look out for safety. 

"Dolphins literally sleep with one eye open."