Humans are involuntary breathers because our breathing is automatically controlled by our brainstem. This means that we breathe without thinking about it and breathe automatically even when we fall asleep.
Whales are voluntary breathers meaning they must think about taking each breath. If they were to fall completely asleep, their brain would rest but they would drown without their brain reminding them to breathe! Instead, they have a fascinating adaptation known as ‘unihemispheric sleep’. To be able to sleep, whales shut down half of their brain at a time. Being partially awake allows them to continue breathing and be aware of their surroundings.
They even close one eye while the other remains alert, quite literally sleeping with one eye open. This behavior is thought to allow breathing, predator avoidance, swimming, and social behaviours all while one half of the brain is resting.