PHShark's don't technically sleep
It was originally believed that sharks don't sleep because they always need to stay moving in order for oxygen-rich water to flow through their gills, which is how they breathe.
However, that theory was disproved when scientists observed how sharks are able to lay still on the ocean floor.
What we have learned, though, is that sharks don't "sleep" the way humans "sleep." There's never a point where a shark's brain enters rest. Similar to how we stay partially awake during "first night" sleep, sharks' brains just take breaks.
(S) bestlifeonline