Here’s what scientists have discovered so far
Sleep in fish may look different than in humans, but a good rest is essential for survival. Here’s what experts have to say about it.

“All animals that have been carefully examined show sleep,” says Philippe Mourrain, a neuroscientist at Stanford University. For instance, in 2019, Mourrain and his colleagues were able to observe zebrafish under a microscope as they snoozed in petri dishes of goo. This allowed the team to measure sleep patterns in the fish for the first time. The patterns looked surprisingly similar to our own deep sleep. The zebrafish even exhibited what looked like rapid-eye movement or REM.

But, do they sleep with their eyes closed?
No, fish do not sleep with their eyes closed because they lack eyelids. This means they cannot close their eyes to indicate sleep. Instead, they have a permanent, transparent eye protector that keeps their eyes open even while resting. This is because fish don’t need to blink or close their eyes to lubricate them, as they live in a moist environment. It might seem strange but it does make sense.

Here on land, animals (including humans) need to blink regularly to lubricate our eyes and stop them from drying out. That’s also part of the reason why most terrestrial beings close their eyes when sleeping.

3 bonus facts about the sleeping habits of fish.

• • Fish have two stages of sleep. This includes “slow-wave sleep” and “paradoxical sleep”, which is often referred to as deep sleep.
  • Fish flick their fins to stay balanced while they sleep. This unique, instinctive behaviour is one of the key tell-tale signs that fish are n the land of nod.
  • Some fish can sleep with half their brain switched off at a time. For those fish that need to keep swimming even when they are asleep, they can put one half of their brain to sleep to keep the other half active! This is known as unihempispheric sleep.

Fish do run a higher risk of being eaten when asleep. Join us on a night dive to find out more.