fisheries science

Fish sleep

 Issue 18 (Sep-Oct 2017)      Bill Brazier


One of the most common questions asked by both non-anglers and anglers alike is whether or not fish actually sleep. “How can they?” many argue, “they don’t have any eyelids!” Of course, nature is a bit more complicated than this – fish, just like any other animal, need rest in order to recuperate, heal, digest food and to grow. Whilst they may not achieve it exactly as we do, fish do, in fact, catch some z’s…

The widely accepted definition of sleep is something along the lines of “the natural periodic suspension of consciousness during which the powers of the body are restored “. However, fish sleep is more akin to a period of inactivity, or torpor, or at least reduced activity where the metabolism is slowed down and energy is conserved. The majority of fish species rest on or near the bottom and appear, for all intents and purposes, to be in a daze or daydream-like state. Some are able to adjust their swim bladders to rest up in the water column, although this is typically risky behaviour, and some have even developed very clever protective barriers, as we will learn later. Most freshwater species, and some marine ones as well, greatly reduce their daily activity in colder water temperatures. This reduction of metabolism is referred to as low-temperature dormancy rather than ‘sleep’. Cold water means more energy is required to carry out daily functions such as finding and digesting food; usually more energy, in fact, than can be gained from low winter food levels. Fish have evolved to know when it’s worth expending energy on finding food and when it’s not and many species will enter a sedentary rest phase in such conditions, often for extended periods of time. As anglers we know that even in the coldest and most unfavourable of conditions, fish can sometimes be caught – and therefore woken from their slumber – so long as you present a bait right next to them. Obviously this depends on the species. Sometimes, though, we have to accept that our quarry is ‘asleep’ and maybe we should do something else instead!

sleeping fish
Most fish species rest on the bottom whilst in their sleep phase and appear for all intents and purposes to be in a daydream-like state. Some even lie on their sides, like this peacock wrasse

How and when do fish sleep

So, do all fish rest in a stationary position when asleep? No! Some fish species that live in the open sea (called pelagic species) must keep swimming 24-7 in order to keep their gills ventilated with oxygen and so do not rest or ‘sleep’ like most other fish. These are termed obligate ram ventilators and examples include great whites and mako sharks, as well as some tuna species and mackerel. Although such fish cannot stop moving, there is evidence to suggest that one half of their brain goes to sleep whilst the other remains active, thus allowing some energy to be conserved whilst remaining alert enough to avoid danger and even feed. After a certain amount of time the side of the brain at rest, switches. This is known as hemispheric slow wave sleep and the same sleeping strategy is also seen in aquatic mammals like dolphins and whales, as well as birds. Despite this, the majority of shark species that live on or near the seafloor, such as our dogfish and tope, are able to rest and remain stationary. Such species keep a constant flow of oxygen over the gills through specialised breathing holes called spiracles located behind the head and basically swallow water constantly to maintain an oxygen flow and prevent suffocation. Rays use this same technique. Although most sharks (and some rays) do have what most refer to as “eyelids”, they do not close them when at rest. The function of these nictitating membranes is simply for protection of the eye when feeding.

great white shark
Some open-sea fish like this great white shark cannot stop swimming and so do not sleep as most other species do

Modern humans typically display a nocturnal sleeping pattern, where we are active by day and sleep at night. Some fish also sleep by night, most notably in the wrasse species such as ballans and cuckoos, as well as the popular American sportfish the smallmouth bass (Micropterus dolomieu), which go into a deep rest cycle during darkness, hiding away and almost completely shutting down. Fish in this night-time sleeping state are said to have a high arousal threshold, meaning they can be poked, stroked and even picked up before they wake up and swim away, as many divers will testify. Rainbow wrasse (Coris julis) of the Mediterranean actually bury themselves in sand at night to keep out of sight from predators. Many wrasse species also show this behaviour, even in aquariums. From an angling point of view, this makes it almost impossible to catch them outside of daylight hours!

“Fish in this night-time sleeping state are said to have a high arousal threshold, meaning they can be poked, stroked and even picked up before they wake up and swim away, as many divers will testify”

Ingeniously, parrotfish (Chlorurus sordidus) form a protective mucus ‘duvet’ or cocoon in which to wrap themselves up in during the wee hours. Excreted from their mouths each night and taking significant amounts of energy, these cocoons were thought to protect the fish from being eaten by predators although research has shown that they act more like a mosquito net to keen pesky parasites at bay. Some wrasse species have also developed this fascinating behaviour. There have been some cases observed of fish eating their cocoons again come morning, to regain some of that spent energy.

sleeping fish
Parrotfish make and sleep in a mucus cocoon to ward off predators and especially parasites whilst they sleep at night

Several fish species have been observed resting in unusual positions, or postures when asleep. Brown bullhead catfish (Ameiurus nebulosus) adopt a typical posture with the tail lying flat on the bottom, fins out-stretched and the body tilted to one side at a 10-30° angle. Many other species lie on their sides. Although undocumented in scientific literature to date, perhaps some freshwater enthusiasts may have noticed juvenile roach (Rutilus rutilus) shoals in a strange motionless, almost vertical “standing” position near the surface during the day, with their tails just under the surface and heads facing towards the lakebed? When doing so they show a high arousal threshold (hard to disturb) and are presumably ‘asleep’. Adult fish resting in the water column tend to have their heads facing slightly upwards instead and perhaps this strange behaviour in juvenile roach (and no doubt the young of other species) is due to still-developing swim bladder control. Even fish that usually engage in routine sleep may skip a few nights if they are migrating or spawning. The very common three-spined stickleback (Gasterosteus aculeatus), for example, will avoid sleep all together whilst their eggs are incubating in their nests so they can use their fins to circulate oxygen to their new babies. Fish can also switch between being day or night sleepers, depending on the time of year and food availability.

Can fish dream?

Fish simply don’t have the same degree of neocortical development as mammals and other higher vertebrates (in fact, they lack a true neocortex, responsible for advanced intelligence) and thus don’t display the brain-wave patterns associated with sleep in a human sense. Fish are also unable to enter REM (rapid eye movement) sleep and therefore cannot dream – their brains are not developed enough for this. However, although opinion still varies, there is some evidence to suggest that fish (just like other animals) use sleep as a means of memory-processing and learning. Some shoaling species are known to require less sleep. Their reliance on other fish for protection makes memory less important, reducing their need to use sleep to store information. On the flip side, fish that need to swim constantly (like those mentioned earlier) live in blue oceanic waters with little in the way of visual information to process and store, and thus need less sleep. This theory is supported by studies on several species of blind Mexican cave fish (Astyanax mexicanus) . Not only have they evolved to have no eyes due to living in total darkness but they also require hardly any sleep, as they have no visual information to interpret.

blind fish
Cavefish have no eyes due to living in total darkness & so need almost no sleep to process the lack of visual information

Can fish go without sleep?

As many of us can testify, sleep deprivation in humans is linked to all sorts of serious issues including lowered immune system response, impaired judgement, erratic behaviour, reduced memory and motor function, mental health disorders and even, in extreme cases, death. In birds and bees is it known to significantly reduce homing and foraging ability, and even plants are negatively impacted when normal day-night light levels are interrupted. Fish also suffer from sleep deprivation.

“Sleep-deprived zebrafish have been shown to perform cognitive tasks (like obtaining food in tanks) less well than fully-rested fish. This likely applies to most, if not all, other species as well”

sleep deprivation fish
Studies on the tiny zebrafish have shown that fish do indeed suffer from sleep deprivation

Melatonin promotes and regulates sleep in most fish, just as it does in humans and other vertebrates. When this hormone production is disturbed, fish show sleep disorders. In fascinating lab studies on zebrafish (Danio rerio), when the fish were kept awake all night they clearly tried to catch up on their sleep (rebound) the next day and were harder to wake up. It has also been shown that fish respond to sleeping medications just as we do. Sleep-deprived zebrafish have been shown to perform cognitive tasks (like obtaining food in tanks) less well than fully-rested fish. This likely applies to most, if not all, other species as well. Although studies are lacking, amazingly fish seem to be able to adapt to long-term sleep deprivation, which is in total contrast to us humans. In the short term, however, it is still likely to have multiple, knock-on effects on all aspects of their behaviour. So, the answer to the question “do fish sleep?” is a resounding yes. They just do it in a way slightly different to people. Even though research on fish sleep is (understandably) thin on the ground  – after all, it’s a pretty difficult thing to study – we have still only scratched the surface on the topic in this article. For sure there is a lot more to learn. If nothing else you now have another excuse to use when you fail to catch anything!

Bill Brazier