Fish, like all living creatures, require periods of rest. This resting state in fish, however, differs significantly from the deep, unconscious sleep experienced by humans and other mammals. Despite their aquatic environment and lack of eyelids, fish enter a sleep-like state vital for their survival, though not “sleep” in the mammalian sense.
The Nature of Fish “Sleep”
Fish do not enter a profound unconscious state comparable to human sleep. Instead, they engage in a “resting state” or “torpor,” characterized by reduced activity and a slower metabolic rate. During this period, their responsiveness to external stimuli decreases, though they remain somewhat aware of their surroundings to detect threats.
Most fish lack eyelids, so they cannot close their eyes during rest. However, behavioral observations like prolonged inactivity, a resting posture, and decreased responsiveness indicate their resting periods. This process is a period of reduced awareness, allowing them to conserve energy and recover without a complete shutdown.
Where and How Fish Rest
Many fish species do rest near or on the substrate, addressing the common query about fish sleeping on the bottom. Bottom-dwelling fish, such as some catfish, gobies, and oscars, commonly settle on the seabed to rest. Other species like the brown bullhead adopt a typical posture with their tail flat on the bottom and body slightly inclined.
Fish position themselves for rest in various ways. Some fish, including angelfish and zebrafish, float motionless in the water column. Others seek out secure spots, wedging themselves into crevices within rocks or plants, a behavior observed in clownfish and some cichlids. Additionally, species such as wrasses, flounders, and sand lances, bury themselves in the sand to rest. Fish maintain these positions by adjusting their buoyancy or finding protected locations that minimize continuous movement.
Diverse Resting Behaviors Among Fish Species
The variety in fish resting behaviors reflects diverse adaptations for survival. Some sharks, known as obligate ram ventilators, must swim continuously to breathe. They enter a state of reduced activity while still moving, sometimes utilizing only one hemisphere of their brain to rest. Conversely, other shark species, like the Port Jackson shark and nurse shark, can rest motionless on the seabed, using specialized respiratory mechanisms to pump water over their gills.
Parrotfish exhibit a unique resting behavior by secreting a mucus cocoon around their bodies before settling for the night. This cocoon serves multiple purposes, including protection from parasites and masking their scent from nocturnal predators like moray eels. Environmental factors such as light-dark cycles, predators, water flow, and food availability influence when and how a fish rests, highlighting the adaptive nature of these behaviors.