Ah, the age-old question of whether or not fish sleep at night. Well, I'm glad you asked! Believe it or not, the answer is both yes and no – depending on what type of fish we're talking about.
Let's start with the 'no' camp first: some species of fish actually stay active during the night, they don't need sleep like humans do! This includes types such as sharks (which are famously awake 24/7) and certain deep-sea anglerfish who have to forage for food in dark waters.
Most bony fishes need oxygen to breathe so must remain near oxygenated water sources even during their sleeping period so can’t simply "shut down." These species lack an actual biological 'sleep state', instead relying on short moments where they reduce their activity levels and become less responsive to stimuli from their environment (such a light changes); this is called aquatic logging or restful swimming.
Now onto those that actually do receive some shut eye: several other species of bony fishes show clear signs of deeper sustained sleep at night when there are fewer competitors for resources such as food (and hence less stress).
It appears that these species will often hunker down in a specific spot known as a ‘sleeping site’ where they noticeably decrease movement despite outside noise disruption or being handled by researchers– this is seen across many different fish species including sea basses, catfish and butterflyfishes.
So while some use their restful swimming technique all day long without ever truly entering into true “sleep mode", others will switch between resting periods during the day with deeper bouts throughout nightly hours like us mammals too!
So yeah - if you've been wondering why your aquarium seems quieter late at night - they may just be catching up on some much needed Zzzzz :)