Have you ever wondered how sharks sleep? These animals are cartilaginous fishes, without a swim bladder (the organ that gives the necessary buoyancy to float) and thus, to adjust their buoyancy, they must continually swim in order not to sink. So it's impossible for sharks to stand still or even swim backwards.
The floating support function is replaced in part by the big liver, filled up of fats, reaching the 25% of the weight of the animal, and the cartilage too. In any case its efficacy is limited and these animals have to resort to an inertial boost to be maintained at the right depth. The continuous movement is used by these animals also to partly ensure a proper breathing.
The sharks then sleep while on the go! However, when is time to rest, they "turn off" a cerebral hemisphere,while keeping the other active. In this way they can continue to swim and rest at the same time, recovering energy.
However, some species of sharks sleep lying on the seabed and pumping water through the gills actively. When resting, the water isn't sucked by the nostrils but by some openings placed near the eyes, called spiracles. But this particular position causes the sharks to intake in their body sand rather than water, with imaginable consequences.