The underwater world undergoes a transformation as day turns to night. Fish exhibit diverse behaviors after dark, moving to specific locations and engaging in activities more complex than simply “sleeping.” Their nighttime routines are shaped by environmental pressures and biological adaptations.
The Concept of Fish Sleep
Fish do not experience sleep in the same way humans do, lacking eyelids and REM sleep. Instead, fish enter a state of reduced activity and metabolism, often called a “rest” or “sleep-like state.” This period involves decreased movement, slower breathing, and reduced responsiveness to external stimuli.
During this resting phase, some fish may remain suspended in the water column, while others settle on the bottom or find sheltered spots. While their eyes remain open, their brain activity shifts, and they become less alert than when active. This restorative process allows them to conserve energy and maintain overall health.
Common Nighttime Locations
Fish utilize various locations for resting or nighttime activity, depending on their habitat and species. Coral reef fish often seek refuge within reef structures, tucking into crevices, under ledges, or deep within coral branches for protection. Some species, like parrotfish, secrete a protective mucus cocoon to mask their scent from predators and deter parasites. Triggerfish might wedge themselves into tight spots by erecting their spines for security.
In the open ocean, some fish undertake a daily vertical migration, descending to deeper waters during the day and ascending to shallower depths at night to feed. Other pelagic species, such as certain sharks, may continue to swim while resting by engaging in “unihemispheric sleep,” where one half of their brain rests while the other remains active. Freshwater fish typically seek cover among dense aquatic vegetation, under submerged logs, or near the bottom of lakes and rivers. Some species, like the Pacific sand lance or certain wrasse, actively bury themselves in sand or mud for protection.
Nocturnal and Diurnal Activity
Fish exhibit distinct patterns of activity, categorizing them as either diurnal (active during the day) or nocturnal (active primarily at night). Many common fish species are diurnal, foraging and engaging in social behaviors during daylight hours before seeking sheltered locations to rest. Conversely, nocturnal fish emerge after sunset, taking advantage of reduced light to hunt and navigate.
Examples of nocturnal hunters include certain catfish species, eels, and knife fish, which possess specialized senses to locate prey in low light. Reef dwellers like cardinalfish, bigeyes, squirrelfish, and soldierfish also become active at night, often preying on invertebrates or other fish. These nocturnal predators use the cover of darkness to ambush unsuspecting prey.
Factors Guiding Nighttime Choices
Fish’s nighttime choices are largely influenced by survival imperatives. Predation avoidance is a primary factor, as many fish seek secure hiding spots to protect themselves from nocturnal predators or to conceal themselves from diurnal hunters. Darkness provides a natural shield, allowing vulnerable species to reduce their exposure.
Food availability also plays a significant role in nighttime activities. Some prey species, such as zooplankton, become more accessible in shallower waters after dark, driving the vertical migration of their predators. Conversely, many nocturnal fish are adapted to forage for food that is more active or exposed under the cover of night. Light levels are crucial environmental cues, triggering the transition between active and resting states and influencing hormones like melatonin, which regulates circadian rhythms. Artificial light at night can disrupt these natural cycles, impacting fish behavior and physiology. Factors like water temperature and currents can also influence where fish choose to spend their night, affecting their comfort and energy conservation.