Step 2 is the hard part.

Matt Ginzton writes here.

How Whales and Dolphins Sleep Without Drowning

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After watching a bunch of aquatic mammals (seals, sea lions, dolphins, and whales) in Patagonia, Vanessa and I got curious about a suddenly obvious question: how do they sleep in the water, when they need to breathe air?

It turns out the answer is different for the cetaceans (whales, dolphins, porpoises) who spend their entire lives in the water, and the pinnipeds (seals and sea lions), which are technically only semi-aquatic.

Back to the real marine mammals, the cetaceans, the answer is a lot more complex than I’d originally suspected — not only do they need to come to the surface to find air to breathe, but their breathing system is under conscious control (unlike, say, humans). So they can’t ever become completely unconscious and continue breathing; also there are times when they can’t stop swimming without sinking. Instead, they continue swimming while sleeping, and are able to sleep with half their brain and body at a time, while the other half remains awake.