Do Dolphins sleep?

Dolphins have to come to the surface regularly in order to get oxygen, which is why we often get to enjoy the sight of these wonderful mammals. But how come they don’t drown when they are asleep? First of all, dolphins don’t sleep for hours in one go like we do; instead they take short 15 to 20 minute naps. Secondly, only half of the their brain goes to sleep each time, so that the other half can remain conscious and keep them breathing.