Marine mammals such as dolphins and whales manage to sleep without drowning, because only half their brain sleeps, while the other half stays awake and handles the breathing and swimming chores. The two hemispheres of their brains work totally independently. For 8 hours, the entire brain is awake. The left side then sleeps for 8 hours. When it wakes up, the right side sleeps for 8 hours. Thus, the marine mammals get 8 hours of sleep without ever having to stop physically.