Room 3 at Roxburgh Area School asks :-
Do fish sleep?
Matt Boyle, an aquarist with Kelly Tarlton's Antarctic Encounter and Underwater World, responded.
Fish do sleep in an energy saving state of what we call resting, some by day some by night. They don't actually have eyelids that close while they are sleeping like humans but they do rest their bodies partially and/or periodically depending on the species.
There are many forms of sleep amongst the animal kingdom. Many sharks don't stop swimming while resting in order to push water through their mouths, and over their gills, to breathe or they will suffocate. Other fish that don't move while resting, still pass water over their gills in order to breathe but have muscular gills to pump water over them.
When fish rest they usually remain still, resting against rocks or bracing themselves with their fins. They will hide in crevices, cracks and under surfaces in order to protect themselves from predators while sleeping. Almost all fish sleep at night except for nocturnal species, which are active at night.
While resting, fish aren't quite asleep but are not fully awake either, most will spend some part of every day resting. When resting they become less aware of their surrounding but never actually become unconscious like humans do. During periods of rest their brain waves never change from being awake. The slightest unusual ripple will disturb a fish and wake them.
Fish like Herring and Tuna rest motionless in the water column during the night while Rockfish and Grouper don't appear to rest at all. Other fish like the Sandagers wrasse will bury themselves in sand while resting, lying on their sides with sometimes one eye peering through the sand. Freshwater fish will usually swim up under logs or riverbanks in order to rest and will do so during the day or night.