Most people, myself included, love a bit of sleep. While we can often get by on varying amounts, it is agreed that sleep is essential to be able to function well the next day; after all a coffee can only go so far!
All mammalian species, from wild animals to our beloved pets enjoy a good nap; curling up in their burrows or hide outs when needed.
But what about our underwater friends, the fish?
How does a fish sleep, if they do at all?
To determine whether or not fish sleep, it is necessary to define what sleep is and what it is needed for.
Yet despite numerous studies, the exact function of sleep remains unknown and is primarily based on supposition. In animals, it appears to be needed to rest and recharge their batteries. The muscles relax, the eye lids close and there is a reduced response to external stimuli.
During this time, the nervous system becomes inactive. This is believed to give the neurons (the important cells which transmit information throughout the body) the chance to shut down and repair.
If animals are deprived of sleep, the ability to function normally each day diminishes. The neurons are not restored, they malfunction and a loss of abilities such as reduced physical performance and memory formation start to manifest.
In short, a lack of sleep can make the best of us struggle to cope.
Looking at Sleep in fish
Fish come in an amazing variety of shapes and sizes; ranging from the Paedocypris progenetica of Indonesia which measures a tiny 7.9mm long to the gigantic whale shark at a staggering 40ft (12 meters).
They can be found in the murky depths of the sea to coral reefs and muddy river bottoms. There is little water that fish of some sort do not inhabit. However fish are much more of a mystery to us than many of their mammalian relatives.
Yet many fish do seem to sleep. Fish lack eye lids, so monitoring this takes more than looking to see if they are having a little shut eye. Instead, it is monitored by a reduction in activity level. Unlike many animals, fish are unable to enter REM sleep meaning that they never have a dream like state of sleep.
With their inability to give us obvious clues such as closing their eyes, sleep analysis in fish has been conducted slightly differently. Variations in physiological mechanisms such as breathing and heart rate are recorded, and both of these show a drop in activity suggesting a reduced metabolic rate. It conserves energy and is believed to allow fish to recharge in much of the same way as people.
With the body slowing down, the responses to external stimuli also become more sluggish. Studies on cavefish showed a slower reaction in fish that had been inactive for 60 seconds beforehand than those that remained on the move, suggesting a sleep like state. Unlike animals, it is not a deep sleep which is handy when you have to be aware of predators trying to snap you up.
But when do fish sleep?
While they are no bird of prey, fish have surprisingly good eyesight. They achieve this by having a rounded lens which enables them to see well underwater. While they normally also have a good sense of smell, this is less reliable due to water currents and as such they rely primarily on their eyesight to find food. [Read Also: Do Fish Drink Water?]
So what does this have to do with sleep? Well as they rely so heavily on their eyesight, this does leave them at a disadvantage during the night. As such, most surface fish are diurnal; active during the day and resting at when darkness falls. Not only does this mean that they can recharge, but they can also lay low to avoid their pesky predators.
Where do they sleep?
With no comfy bed to curl up in, fish come up with ingenious ways to get some kip. Some, like the sand-sleeping wrasses, bury themselves in the substrate to sleep. The parrot fish makes its own duvet, creating a bed of their own mucous to cocoon themselves in just before sunset. Others wedge themselves in between rocks or corals. By hiding themselves while sleeping, they can do their best to avoid night time predators such as sharks which hunt them down using their highly developed and sophisticated sense of smell.
Did you know?
While we know little about fish, even less is known about sharks and their ability to sleep. As a ram-ventilation species (i.e. they need water constantly moving over their gills in order to extract the oxygen needed to survive) many are unable to stop swimming. They may sleep in a way similar to dolphins, shutting down one hemisphere of their brain for a few minutes at a time, but again this is all speculation.
Fish can suffer from sleep deprivation just like us. Researchers working with zebra fish kept prodding them to keep them awake at night. Just like people, the zebra fish then ended up napping as soon as they had the chance – even during the daytime which they would normally avoid.
Despite fish being unable to demonstrate some of the signals that signify sleep in mammalian species, there is no denying that they do rest routinely. This reduced response to stimuli is the greatest signal that they are indeed, switching off and do get some nap time in; just like people.