Many fish assume typical sleeping positions at night. They go to rest and sleep. This is done, for example, by tetras, armored catfish and gouramis. Because most fish don’t have eyelids, they can’t close their eyes and don’t appear to be sleeping. But they sleep.
Corydoras schwartzii like to lie down on a sheltered leaf to sleep. Gouramis often burrow into a plant to sleep.
If the aquarium lights are switched on again in the middle of the night, these fish are visibly irritated and need some time before they even come out. The lighting should therefore not be switched on at night without good reason.
Other animals are constantly active. E.g. zebrafish often swim constantly through the aquarium at night.
There are also fish that are predominantly nocturnal or for whom many aquarium lights are too bright. These include, for example, African glass catfish. These are in the shade under plants during the day. Seconds after you turn off the lights, they come out and race through the aquarium all night. Apparently they can also find their way around in the dark.
Fish have an internal clock. If the lighting times are extended, the fish will initially go to sleep at the usual time, even with full lighting.
Some fish need about half an hour to wake up. Only then should you feed them.