Fish not only sleep with their eyes open, they will not close their eyes at any time, because this animal has no eyelids. Eyelids are important for land animals because they are needed to moisten the eyeball and protect it. Fish live in the water, their eyes are wet, so the eyelids are of little significance. Some sharks have an eyelid-like nictitating membrane that covers their eyes when they are swimming at high speed to catch food.
1. Hiding in the dark: Many fish will hide in the dark when they sleep, such as goldfish, carp and so on. Some of them will hide in the aquatic plants, while others will mostly be in the crevices of stones. The purpose of doing so is to avoid some enemy organisms. Even domesticated fish will maintain this habit.
2. Get into the sand: Some demersal fish get into the sand when they sleep, so they can hide themselves from other aquatic animals.
3. Bubble wrap: Some fish can secrete a layer of gelatinous substance when they sleep, such as split mouth fish, this substance will harden when it meets water, which can resist the attack of some animals, and it can also leave a mouth to facilitate its breathing while sleeping.
4. Swimming and sleeping: Some fish with a strong metabolism must keep swimming all the time, except when they sleep, otherwise they will die, so they will sleep while swimming, and they can't tell whether they are sleeping or swimming.
