Do fishes sleep?

The answer may surprise you!

It has probably been a wonder of many, whether or not fish sleep or take breaks. Simply put, the answer is yes, fish do indeed sleep. However, they do so with their eyes open due to not having eyelids. Fishes experience a period of reduced activity and metabolism which performs similar restorative functions that nocturnal sleeps does for us humans.

How do fishes sleep?

In a myriad of ways! They sometimes do it in the day as they don’t show the characteristic brainwave patterns like REM sleep present in humans, and some, including most sharks, have to continuously swim in their sleep. As fish also enter a period of reduced activity and metabolism to experience restorative functions, some species are more obvious about it than others.

Some rest on the bottom or in coral crevices, while the parrotfish, for instance, would secrete a mucus “sleeping bag” around themselves before sleeping. If you get up quietly in the middle of the night, chances are you will find your goldfish in a trance-like state, hovering near the tank’s bottom making just minimal corrective motions with its fins to maintain its position in the water column. Feeding fish when they’re in this state will take them a noticeably longer than usual time to respond.

How can you tell if a fish is asleep?

In human terms, fish do not have typical day/night lives that we recognise via regular 9 to 5 routines. Some fish are active by day, others by night, while certain deep ocean fish appear to never stop moving (often, sharks befall similar fates, having to constantly move to keep water and oxygen flowing through their gills).

‘Shut eye’ is not impossible for most fish, say for sharks, who possess eyelids. However, what happens is most fish ‘switch off’ when it is safe to conserve energy, and enter a trance-like daydream state. This is visible in fish tanks or aquariums – some float motionless in place, some float nearer the bottom of the tanks, some build nests or nestle themselves into the seabed or coral for safety and camouflage reasons. Interestingly, research has shown that the zebra fish even experiences sleepless nights if faced with annoyance! Sleep deprivation or insomnia are common symptoms faced by zebra fish under duress- but they were found to catch up on sleep as soon as the opportunity arose.​

Ultimately, it need not worry you if your fish chooses to sleep during the day or night. Either way, fishes definitely ensure that they are receiving ample rest!

Adapted from Pet Lovers Centre Magazine – Issue 11

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