Sperm whales sleep together in a pod facing up in the water. From bioGraphic:

Photographer Franco Banfi and his fellow divers were following this pod of sperm whales (Physeter macrocephalus) when the giants suddenly seemed to fall into a vertical slumber. This phenomenon was first studied in 2008, when a team of biologists from the UK and Japan inadvertently drifted into a group of non-responsive sperm whales floating just below the surface. Baffled by the behavior, the scientists analyzed data from tagged whales and discovered that these massive marine mammals spend about 7 percent of their time taking short (6- to 24-minute) rests in this shallow vertical position. Scientists think these brief naps may, in fact, be the only time the whales sleep.