Note that this is only my opinion, and should not be treated as a definate answer.

First, before I begin, it is only necessary that readers need to know the background of the topic and how it affects fish (and other pets, too).

Sleep(v)- a state of consciousness where the body ceases all invouluntary functions (ex:moving fin, swimming around, eating food, etc.), while running on voluntary functions (ex:breathing, heart beating, keeping balance, gills flapping, etc.) in order to save energy and promote rest.

It is true that fish do not have eyelids, so we cannot tell if they are sleeping because they do not appear to have closed eyes, like we do. However, fish do seem inactive, like we are during sleep, and they do seek out shelter (ex: a rock, plants, etc.). Usually, they are motionless while resting and their color seems to be a bit dull.

I have also examined my own goldfish when they were sleeping on different days (being careful not to wake them up) and noticed they posess the characteristics mentioned above. Also, when the aquarium tank light is on, it is easier for them to wake up and harder for them to sleep. Sometimes, my fish fell into what we would call "deep sleep", and at that point they ignored some disturbances, like tapping on the glass and noises that they would usually get scared from.

Fish also have sleeping positions that relate to ours. For instance, fish sleep on their sides and stomach, just not their "back".

It turns out that the way fish sleep is more similar to ours than we think and as we research, we can find many similarities between us, and our fish.

Submitted by Warzone