Few things are more relaxing than floating in a pool, hearing the sound of water, and resting your eyes for just a little while. For whales and dolphins, however, sleeping in the water is a bit more complicated. Indeed, whales do not have the luxury of floating on pool beds, and the sea is pretty dangerous when predators lurk in the dark. Unlike us humans, whales lack an unconscious breathing system. Cetaceans are voluntary breathers, which means that they need to think about every breath they take, ²⁴⁄₇. So how do these air-breathing mammals survive a night underwater? Short response: whales sleep with half their brain only!