Do fish sleep?
Since the second night we have had our clownish, after the lights go off at 9 p.m., they retreat to the front right corner of the tank and begin the nightly process of cleaning out their “bed”. The larger of the clownfish stirs up the sand with her body and tail as if to deepen and clear any debris from the depression they have dug. The small clownfish then mimics her, as if he is learning the process, and they do this off and on for a couple of hours.
Watching for several nights now has me asking, “Do fish sleep?” If not, why else would they do this every night after the lights go off? Also, most nights I wake up to let our dogs out around 2:30am and find both clownfish snuggled close inside this crater, leaning against the glass. They move very little during this time, and if they had eyelids I would guess they would be closed.
I thought this question would be easy to answer, but upon searching found it is very debatable. Some aquarists say yes, and some say no. So I looked at articles on the subject and found some say yes, and others no, depending on how the word “sleep” is defined. So, to answer this for myself, I looked at the definition of sleep. According to the Merriam-Webster dictionary sleep is defined as “the natural periodic suspension of consciousness during which the powers of the body are restored.” Because fish have to be on constant alert for predators, I’m sure they do not completely “suspend consciousness” as in the definition of sleep. They do however, need to restore their body and thus need “rest”. So the word “sleep” may not be the most appropriate word for what fish do. As far as fish moving while in this state of rest, some fish, like sharks, need to move constantly in order to move water through their mouths to breath.
It’s easy for me to think of my fish as sleeping when I find them snuggling and leaning against the glass at 2:30 in the morning, but I don’t think they are actually sleeping, as humans would sleep. I think they are, however, resting, awaiting for the daylight when they can explore the tank once again.