Fish Sleep

Although sleep in fish has not been extensively studied, it is well known that fish do indeed sleep, although not in the same way as we do. In fact most fish simply ’switch off’ when safe to do so in order to save energy and go into daydream state. When fish do have this period of reduced activity and metabolism, it seems to perform the same restorative functions as sleep does in humans, but it is still not know if fish actually dream? However, just to confuse things somewhat further, species that always live in open-water shoals or that swim continuously (because of a need for ram ventilation of the gills, including most sharks for example) are suspected never to sleep and have to keep swimming. Likewise, these sleep patterns are easily disrupted and may even disappear altogether during periods of migration, spawning, and parental care.

As fish do not have eyelids to close, it is not always obvious as to when they are in this state as they sometimes do it during the day as well as night. One thing we do know, is that fish do not exhibit the characteristic brainwave patterns like 'REM sleep' (Rapid Eye Movement sleep is a normal stage of sleep characterised by the rapid and random movement of the eyes) as seen in humans. 

When sleep can be observed in an aquarium, the inhabitants appear to float motionless midwater, within plants, crevices in rocks and ornaments, or simply resting near the bottom of their aquarium and usually occurs when the lights are off. The fish simply make just the minimum of corrective motions with their fins to maintain their position in the water column. Some species (such as wrasse and parrotfish) specifically secrete a mucus “sleeping bag” around themselves before they go to sleep and many fish change colour (becoming much darker) so to camouflage themselves into their surroundings to avoid predation. In fact some species appear to go into such as deep state of rest (e.g. Spanish hogfish and blue-headed wrasse) that they can even be lifted all the way to the surface without evoking a response. 

Sometimes this sleeping behaviour can also be observed when you put food into an aquarium and the fish take noticeably longer than usual to respond, as if they have trouble waking up. Research has also shown that fish can suffer from sleep deprivation and insomnia. By repeatedly disturbing Zebra fish, researchers were able to keep the fish awake at night. Those fish that had suffered a disturbed night were found to catch up on their sleep as soon as the opportunity arose.