HOW FISH SLEEP DEPENDS ON THE SPECIES. Nocturnal fishes, like cardinalfishes and those closely packed platoons of grunts and gray snappers you see just hanging around by coralheads and pier pilings during day dives, are likely resting rather than actually sleeping.
Some species, like parrotfish, clearly sleep at night, although they seem like they can be awakened with minor disturbances (I base this on my own clumsiness around them on night dives).
Some, like bluehead wrasses and Spanish hogfishes, are such sound sleepers you can handle them – as in, lifting them to the surface – without waking them up (Just to be clear, you shouldn’t do that).
From personal experience, trumpetfishes fit into this category. I have a dive buddy who likes to pet things underwater. Once, he demonstrated this by standing (or appearing to stand) a vertical, totally zonked out trumpetfish upright on his hand during a nighttime pier dive (Just to be clear, he didn’t actually handle the fish).
EYES WIDE OPEN, STEADYING TAILS
It’s hard to judge fish sleep, since they don’t have eyelids; for them, getting shuteye is literally impossible. REM sleep is not in their game plan. Actually, there doesn’t appear to be a massive amount of research on fish sleep, and a lot of it is about zebrafish.
What is clear is that most fishes go through a period sometime each day when they power down, conserve energy with slowed heartbeat and less movement, and restore themselves. Their gill movements slow down.
For some, it may be more of sort of daydreaming state than actual sleep – a sort of suspended animation. Those schools of grunts appear as much to be awake, alert to danger, as to be in a resting phase. And fish that sleep or rest hovering in the water column seem to be able to remain steady with subtle tail movements.
HOLE IN THE WALL GANG
And, anyway, if you’re a daytime fish wanting to avoid nighttime predators, hiding out in a hole of some sort is a good move. Some stuff themselves into crevices or vase or barrel sponges. As dusk falls, little chromis and damsels hover closer and closer to the corals, finally simply disappearing into reef crevices. Some simply hide out among the branches of coral, gorgonians or rope sponges. Many wrasses bury themselves in the sand for the night.
On the other hand, some fishes just hide out in the open. Parrotfish are renowned for enveloping themselves in mucus cocoons, presumably masking their scents from predators, sometimes seen in the open. They often do, whether in the open or hidden away, but not always.
SOME FISH SLEEP, SOME DON’T
A theory holds that the purpose of sleep is to allow animals to process visual images from their day. If true, that might explain why some blind cave fish are known not to sleep – they don’t have any visual images to process.
Tunas and some sharks are said not to sleep, since they have to swim continuously to maintain gill flow. According to the visual imagery theory, open-ocean-swimming tunas, for example, also lack much in the way of visual imagery to contemplate.
But species of continuously swimming sharks are believed to “sleep swim,” in which parts of their brains rest while they remain in motion. Dolphins –marine mammals, not fishes – are known to be able to shut down one half of their brains at a time in order to rejuvenate while continuing to swim. Tuna may have the same capabilities.
And for some species sleep goes out the window during specific periods of reproduction and/or brooding eggs.
PRINCIPAL SOURCES: Reef Fish Identification Florida, Caribbean, Bahamas, Paul Humann, Ned DeLoach; Reef Fish Identification, Tropical Pacific, Gerald Allen, Roger Steene, Paul Humann, Ned DeLoach; Reef Fish Behavior, Ned DeLoach; The Coral Reef at Night , Joseph Levine, Jeffrey Rotman; “How Do Fish Sleep: Everything You Need To Know,” Fishkeeping World; “Do Fish Sleep?” NOAA; “Do Fishes Sleep?” Australian Museum; “Do Sharks Ever Sleep, and How?” ThoughtCo.; “Sleep in Fish,” Wikipedia.com.