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If you’ve ever owned a fish tank, you’ve likely noticed that your fish swim back and forth endlessly during the day. This seems to support the common belief that fish sleep in the same way humans do, by shutting down one hemisphere of their brain at a time as they alternate between sleep phases. However, there’s also plenty of anecdotal evidence from experienced aquarists that this isn’t true. We explore whether fish really sleep and what experiments have revealed about their sleeping habits.
Do all fish really sleep at night?
Most fish do really sleep during daylight hours, but not all of them. The fact is that fish don’t have eyelids to close over their eyes and block out light, so if they are going to sleep, it has to be at night. Some fish live in places where there is never total darkness, such as coral reefs that are bathed in moonlight or lake bottoms under bright street lights. These fish sleep for short periods of time and even then only when there is no movement around them. Other species are able to rest (with half-closed eyes) even in full sunlight. Sleeping may mean resting on a sand or mud bottom with reduced activity, though some species will swim about periodically until ready to start hunting again.
How do they catch up on their ZZZs?
If fish sleep, how do they catch up on their ZZZs? Find out why it’s not as simple as closing your eyes and counting sheep. Before delving into the answer to that question, let’s quickly explore whether fish even have dreams (or nightmares) in order to provide some context for understanding our deep-sea denizens’ potential slumber habits. So what do you think – are fish capable of dreaming? Yes or no? As always, I encourage readers to chime in with their thoughts by leaving a comment below; discussion is an essential part of learning!
What do they dream about?
Despite spending a third of their lives sleeping, it is not known whether fish sleep in a manner similar to humans or other mammals. While some fish certainly do appear to sleep, others remain active and only rest as they rise to the surface for a gulp of air every few minutes. This raises two questions: how can we tell if an animal is asleep and what do animals that sleep underwater dream about? Why do dolphins sleep with one eye open?: It turns out dolphins need to use up brainpower to keep themselves alive – so when dozing off they choose one half of their brain at random (half asleep!) just like we close one eye while rolling over on our side. Although some dolphins are able to stay below water for hours, most tend to come up every couple of minutes for fresh air, especially when resting horizontally on the water’s surface with eyes shut.
Do some types of tropical fish never stop swimming around?
Do fish sleep is a question that often sparks debate. In short, yes, they do sleep. Tropical fish exhibit circadian rhythms that are similar to those of humans and other mammals—meaning they’re governed by daily cycles. Most tropical fish rest during daytime hours, typically in shaded areas or on sunken tree branches to avoid predators lurking below.
Do fish really sleep when it’s dark out?
Like humans, fish do sleep. But they need less sleep than we do and they can go longer without it. Unlike humans, fish don’t require hours of uninterrupted time to get their needed ZZZs in. In fact, many species only need a few minutes of rest in order to feel refreshed enough to get back up on their fins. However, just because they can get by with shorter periods of slumber doesn’t mean they always do. Some studies have shown that some fish occasionally spend upwards of 20% of their day resting in a state similar to hibernation or anestrus (the period between gestation periods). It’s not clear whether or not that has any physiological benefits for them as adults—or even if certain kinds prefer one state over another—but for now, there isn’t much research about fish sleeping habits.
How long can they go without shut-eye?
Despite popular belief, fish do not sleep in one continuous chunk and come out of it rested. Scientists have observed fish taking several brief breaks—called sleeps—during a 24-hour period, but these are not in any way similar to what humans experience. While mammals sleep for longer stretches of time, fish are considered continuous sleepers. This means they never go through cycles of active sleep and restorative slow-wave (non-REM) sleep like humans do. What’s more, scientists don’t really know why fish need to catch some zzzs at all. Some believe that their unusual sleeping patterns might help them conserve energy during their sensitive early life stages when food is scarce. In adults, however, there is still plenty of research needed before scientists can draw conclusions about why fish slumber and if there are benefits associated with getting enough sleep as an adult finned friend.
The old adage early to bed, early to rise… doesn’t apply here.
Sleeping is extremely important to humans, but does it really matter for fish? There have been some interesting studies about fish sleeping habits, including what’s commonly believed to be their ability to shut down half of their brain at a time. They’re nocturnal animals, after all. But even if they do get shut-eye, they don’t sleep like we do and therefore probably don’t dream like we do either. Here are some fun facts about fish sleeping habits and why scientists keep trying to learn more about them. (Although answers remain elusive.)
