Whales and dolphins sleep with only half of their brain at a time.

Whales and dolphins are mammals, and that means they breathe air into lungs just like we do. So without water-breathing evolutionary advantages like gills, how do whales and dolphins sleep without drowning? One big benefit is that marine mammals are much better at holding their breath underwater than us primates. Sperm whales (Physeter macrocephalus), for example, can go 90 minutes without having to come up for air. But over millions of years, whales and dolphins have also developed a few strategies for catching some z’s on the go. Young cetaceans, for example, will swim alongside their mothers and rest within her slipstream. This gives calves time to develop sleeping strategies as well as put on enough blubber to keep them afloat. As adults, whales and dolphins will sleep with half of their brain still operating at a low level of alertness. The other half, along with the opposite eye, is completely asleep. This helps a cetacean keep one eye out for predators, while also periodically breaching the water’s surface to take in gulps of air through its blowhole.