What Sharks Are Really Doing at Night

In the dark reefs off Florida, a sand tiger shark was tracked gliding for hours at midnight without ever closing its eyes. Unlike most fish, the sand tiger can’t stop swimming, or it’ll suffocate. So how do sharks like this rest? Researchers found some species enter a state called unihemispheric slow-wave sleep, keeping half their brain awake to keep moving and breathing. That’s why you’ll never see a shark truly asleep the way you imagine, and why the sand tiger shark’s midnight swim is rest and survival all at once.