The reason why fish sleep with their eyes open is that this animal has no eyelids, so it is not only sleeping, but also keeping its eyes open at any other time. Eyelids are common in terrestrial animals because they are needed to keep the eyeball moist and to protect the eyes. Only some sharks among the fish have nictitating membranes, similar to eyelids, which can cover their eyes at high speed to avoid injury.
1. Hiding in the dark: Because fish may be attacked by enemy organisms when sleeping, they tend to choose some relatively safe places to hide, like goldfish sleeping, often near stones or water plants. There are also demersal fish that can hide themselves in the sand at the bottom of the water.
2, secretion of bubbles: Some fish in order to protect themselves while sleeping, will secrete a gelatinous substance, use it to wrap themselves up. There is no need to worry that bubbles will suffocate themselves, because it will leave a hole in the mouth to facilitate its water exchange and breathing.
3. Sleeping while swimming: Some fish in the ocean have a high metabolism and must keep swimming all the time, so they will not stop when they sleep. For example, sharks, tuna and so on, they also swim when they sleep, so it is difficult to tell whether they are awake or not.
