The world of underwater slumber is unlike what we see on the surface. For mammals (哺乳动物), a good night's sleep sends an animal into a state of unconsciousness (无知觉) during hours of restorative sleeping that helps heal the body and solidify memories firmly in the mind. For fish, however, sleep isn't such a well-defined process. Instead, many fish species have taken to spending short periods in a state of reduced activity or rest while keeping their eyes open to scan the water for potential threats. During these periods of reduced activity, a fish's bodily functions slow down and its watchfulness drops.
The breadth of animals in the ocean that exhibit this type of reduced activity is wide-reaching. Among the hundreds of thousands of creatures in the sea, some merely ride along with the current, such as jellyfish. Others find rocky bedrooms for the night, while those such as stingrays bury themselves in the sand to rest. However, it must be really hard to get a good night's sleep when you breathe air but live in water. This is something that the world's underwater mammals have to deal with.
To prevent themselves from drowning (溺死) in their sleep, underwater mammals such as dolphins and manatees have developed the ability to "turn off" one side of their brain to rest, known as unihemispheric sleep. While one side is fast asleep, the other keeps all the animal's vital functions running, such as breathing. Manatees, for example, love to sleep, spending up to 12 hours a day in a deep slumber. However, as air-breathing animals, manatees need to surface every 20 minutes or so for oxygen. To achieve this, the half of the brain that's still awake sends the sleeping manatee to the surface to get some air.