By Brasen Tham (Y12)
Welcome back to Useless Genius! This week’s mildly suspicious question: Do fish sleep?
At some point in life, you must have stared at a fish tank and wondered whether fish ever sleep. After all, have you ever seen a fish close their eyes or yawn? The surprising answer is that fish aren’t permanently awake and do sleep, but just not in the same way humans do.
The main reason fish look as though they never sleep is that they do not have eyelids. Humans and most land animals close their eyes when they are asleep, which makes it obvious that they are resting. Fish cannot do that, so even when they are sleeping, they appear fully awake.
Scientists have studied fish behaviour and found that many species enter their resting state when they move little, stay in one place and react more slowly to what is happening around them. However, a few species of fish, especially certain sharks, need to keep moving even when they are resting so that water continues to flow through their gills.
Fish sleeping habits can also be surprisingly unusual. For example, parrotfish protect themselves during the night by producing a thin layer of mucus around their bodies, which hides their scent from predators when they sleep.
So the next time you see a fish floating in the same spot for a long time, it is probably not bored and is not broken. It may simply be resting in its own way. Fish do sleep, but without pillows, blankets or closed eyes.