How do sharks obtain oxygen while they sleep?

Most sharks must keep water constantly moving across their gills in order to obtain oxygen. This means they must keep swimming, even while sleeping, in order to breathe. There are a few exceptions to this rule. Some species of sharks, such as the nurse shark and the wobbegong shark, can pump water across their gills without having to swim. Other species of sharks, such as the great hammerhead shark and the bull shark, can breathe air by gulping it in through their mouths and then expelling it through their gills.