|MadSci Network: Zoology|
Sharks don't sleep as we know it but they do rest. Often they will come to a quiet bottom area and stay there motionless. Their gills keep moving water and oxygen but they are "sleeping" motionless on the bottom. If you splash the water or create a concussion wave near them by moving rapidly, they seem to "awaken" rapidly and move off. Hope that helps.
Try the links in the MadSci Library for more information on Zoology.