if you ever see a shark documentary on, watch it, they've got some good pics of shark sleeping most of the time.
basically, sharks that have to keep moving go into huge swimming circles (think circling vultures underwater) and swim around in circles while they sleep.
it's a simple exercise, and they can easily pull it off while taking a nap, so, thats how they sleep.
as for when, no clue, depends on the shark
