Melissa here. Have you ever wondered if fish actually sleep? Well, wonder no more. They do in fact sleep, just not the way we typically know of it. Fish do not have eyelids to shut when they sleep do nor do they have characteristic brainwave patterns like REM sleep. Fish do, however, appear to have periods of reduced activity and metabolism.
Many reef fish like damsels and clownfish frolic around during the day, then tuck themselves into a crevice or cave to sleep at night. Parrot fish have an odd way of tucking themselves in bed: they secrete mucus to make a little seeping bag for themselves. Some wrasses tuck themselves under the sand while others are like parrot fish and encase themselves in mucus.
I used to have a clownfish that laid on his left side in the back corner of my tank. The first time I saw him after the lights went off laying on his side I thought he was dead. When I approached the tank I must have startled him enough to make him swim erratically for a few moments then act normal. My blue devil damsels have a particular rock they like to sleep in. As soon as the lights go out you can see both devil damsels turn dark in color and head into their rock.
I am sure many of you reading this have seen interesting behaviors with your fish when the lights go out. Please feel free to share.