Do dolphins and whales play?
Dolphins play a lot, especially young dolphins. They spend many hours per day balancing objects such as stones, seaweed, and fish parts on their snouts and flippers, herding fish, and pushing objects with their snouts. It is common to see youngsters in the juvenile pods chasing each other, nipping at each other's fins, finding and sharing objects, and even blowing bubbles. Scientists believe that play behavior teaches the young dolphins the skills they will need to socialize, hunt, mate, and survive as adults. Even as adults, dolphins never seem to lose their interest in playing.
The largest members of the dolphin family, killer whales (orcas), also have been observed at play--playing with their food! Ingrid Visser, a student at the University of Auckland, watched as 19 orcas pursued 55 stingrays. Plowing into them from below, the orcas tossed the stingrays up into the air and batted to other orcas like Frisbees. She theorized that maybe the orcas were trying to stun the stingrays so they wouldn't sting when they swallowed them.
(source: "Playing with their food?" National Wildlife, Feb/March, 1998
Do whales sleep?
Some scientists believe that whales "catnap" so that at least part of their brain is aware of what is going on around them. Since whales and dolphins have to breathe consciously (humans are "unconscious" breathers-we breathe even in our sleep), they also have to wake up to take a breath.
Some scientists have postulated that dolphins and whales sleep with half a brain -- half their brain rests while the other half stays alert. Recently, though, other researchers have come to doubt this theory.