In human cognition, closing the eyes is often a sign of sleep. However, when we observe fish, we find that even when they are resting, their eyes seem to remain open. Does this mean that fish don't sleep? Or do they simply not need to close their eyes? This article will systematically analyze why fish "sleep with their eyes open" from zoological and neurophysiological perspectives.
Do fish really sleep?
1.1 Definition of "Sleep" in Fish
Although fish do not exhibit obvious eye-closing behavior, they do have a sleep-like state, scientifically known as "behavioral hibernation" or "resting state."
Activities reduced or stopped
Slow response to external stimuli
Slowed metabolism
This state meets the basic behavioral standards for sleep.
1.2 Sleep cycles are less distinct in mammals.
Unlike humans, fish do not have a clear REM (Rapid Eye Movement) and non-REM phase, but EEG studies have shown that some fish exhibit rhythmic hibernation at night.
2. Why do fish sleep with their eyes open?
2.1 Fish do not have eyelids
Most fish are born without eyelids , so their eyes appear open even when they are resting or sleeping.
2.2 Adaptability to continuous eye exposure
Because they live in water, fish's eyes are constantly moistened by water, so they don't need to rely on eyelids to prevent dryness like terrestrial animals .
2.3 Evolutionary Advantages of Alertness
As fish are prey for many predators, keeping their eyes "open" may have evolved as a form of camouflage, making it impossible for predators to determine whether they are in a dormant state.
III. The sleeping patterns of fish vary depending on the species.
3.1 Daytime active vs. nighttime active
Fish that are active during the day (such as goldfish): reduce their activity and enter a dormant state at night.
Nocturnal fish (such as some catfish): enter a state of inactivity during the day.
3.2 Fixed sleeping posture and hiding behavior
Many fish do the following when they sleep:
Staying on the bottom of the water, in rock crevices, and among aquatic plants
Change body color to disguise oneself
Reduce gill movement frequency
3.3 Special Sleep Phenomena
Parrotfish : They secrete a sleep cocoon resembling a "mucus wrapper" at night, which may have a parasite-repellent effect.
Sharks : Some species sleep while swimming, maintaining breathing and propulsion through muscle reflexes.
IV. The Neural and Physiological Mechanisms of Sleep in Fish
4.1 Brain activity rhythm
Although fish lack a cerebral cortex, their hypothalamus and midbrain regions exhibit rhythmic activity changes during sleep.
4.2 Hormone involvement
Melatonin also plays a role in regulating circadian rhythms in fish, causing them to enter a dormant state when light levels decrease.
4.3 Gene Regulation
Studies have shown that in model organisms such as zebrafish, the expression levels of certain sleep-related genes (such as hypocretin) decrease during hibernation.
V. The Influence of the Rearing Environment on Fish Sleep
5.1 Disruption of light rhythm
Irregular artificial lighting can disrupt the physiological rhythm of fish, leading to decreased sleep quality.
5.2 Lack of concealment
Without areas to hide and rest, fish struggle to enter a deep state of hibernation and are prone to stress.
5.3 Influence of Water Flow and Sound
Continuous strong water currents or vibrations can disrupt fish's rest cycles, affecting their health and behavioral stability.
VI. Conclusion: Opening one's eyes does not equate to being awake; even in deep water, one may be asleep.
Although fish appear to have their eyes open, they do need and have their own ways of resting. This sleep pattern, so different from that of humans, reflects the diverse adaptations that organisms have evolved in their environment. Understanding fish sleep not only contributes to scientific research but also helps us provide healthier and more natural environments for them in aquariums.
