海豚是如何睡觉的？

海豚在很多方面都和人类很像。比如说海豚有和人类很像的特殊的呼吸系统，唯一的不同是海豚能不呼吸氧气在水下呆很长时间。但是海豚使如何做到睡觉不溺水呢？
Since Dolphins are like human being in so many ways. For example, dolphins have a special respiratory system  just like humans. The only difference is that dolphins are able to stay underwater for a long time without breathing oxygen. But how do dolphins manage to sleep without drowning?

Generally, dolphins sleep with only one brain hemisphere in slow-wave sleep at a time, thus maintaining enough consciousness to breathe and to watch for possible predators and other threats. Earlier sleep stages can occur simultaneously in both hemispheres. In captivity, dolphins seemingly enter a fully asleep state where both eyes are closed and there is no response to mild external stimuli. In this case, respiration is automatic; a tail kick reflex keeps the blowhole above the water if necessary. Anesthetized dolphins initially show a tail kick reflex. Though a similar state has been observed with wild sperm whales, it is not known if dolphins in the wild reach this state. The Indus river dolphin has a sleep method that is different from that of other dolphin species. Living in water with strong currents and potentially dangerous floating debris, it must swim continuously to avoid injury. As a result, this species sleeps in very short bursts which last between 4 and 60 seconds.