The immense, three-dimensional ocean presents a unique set of challenges for the fundamental biological necessity of sleep. For marine life, this deep need for rest must contend with the constant threats of predation, the necessity of maintaining buoyancy, and the absolute requirement to breathe in a water-based environment. Unlike terrestrial animals that can simply lie down and disengage from consciousness, many sea creatures must remain partially alert or in motion to survive. The diverse solutions to this dilemma—from half-brain sleeping to protective mucous barriers—represent some of nature’s most fascinating evolutionary adaptations.
Defining Rest Underwater
Sleep in the ocean is often a state of reduced activity and responsiveness rather than the deep unconsciousness seen in land mammals. This resting state is characterized by a lowered metabolic rate, decreased heart rate, and an increased threshold for arousal by external stimuli.
To compensate for vulnerability, many marine animals employ specific safety measures when they rest. This often includes seeking out physical shelter, such as coral crevices, rocky overhangs, or burying themselves in the sand. Other species rely on effective camouflage or grouping together in large numbers to reduce the individual risk of attack while their awareness is lowered.
How Bony Fish Find Repose
Bony fish, or teleosts, have developed various behavioral strategies to enter a state of repose without ever fully losing awareness. Most species lack eyelids, meaning their eyes remain open even when they are resting, contributing to a state of reduced awareness rather than true sleep. Their resting posture often involves finding a sheltered location, sinking to the bottom, or remaining suspended almost motionless in the water column.
A particularly unique adaptation is seen in certain reef fish, like parrotfish, which construct a protective mucous cocoon each night. The fish secretes a transparent, full-body envelope over a period of up to an hour, which acts as a physical barrier. While once thought to deter large predators, research suggests this cocoon primarily functions like a mosquito net, shielding the sleeping fish from blood-sucking ectoparasites called gnathiid isopods. This nightly ritual is energetically costly, accounting for about 2.5% of the parrotfish’s daily energy budget, yet it provides a significant defense against infestation.
Sleep Adaptations of Marine Mammals
Marine mammals, such as dolphins and whales (cetaceans), face a profound challenge because they are voluntary breathers, meaning they must consciously decide to take each breath. If they were to fall completely unconscious, their involuntary breathing reflexes would not be sufficient to prevent them from drowning. This constraint led to the evolution of a highly specialized sleep state known as unihemispheric slow-wave sleep (USWS).
USWS allows one cerebral hemisphere of the brain to enter a deep sleep state while the other half remains awake and vigilant. The awake half of the brain controls the necessary processes, like surfacing to breathe and keeping the animal in motion, while the sleeping half receives restorative rest. This half-asleep state is often visible because the eye contralateral to the sleeping hemisphere typically closes, while the eye connected to the active brain half remains open.
The two hemispheres alternate their sleeping periods, with each side of a bottlenose dolphin’s brain receiving about four hours of slow-wave sleep per day. During this time, cetaceans often engage in “logging,” where they float motionless at the surface, or swim slowly in a coordinated group. This continuous vigilance explains why cetaceans exhibit little to no rapid eye movement (REM) sleep, as the muscle atonia and impaired thermoregulation associated with REM sleep would be maladaptive in the cold, aquatic environment.
Navigating Rest Without Stopping: Sharks and Rays
The resting behavior of elasmobranchs, the group that includes sharks and rays, is primarily determined by their method of respiration. All sharks must move oxygen-rich water over their gills, a process that can be accomplished in two ways. Some species, known as obligate ram ventilators, must swim continuously with their mouths open to force water across the gills, a method called ram ventilation.
For these obligate ram ventilators, such as the great white shark, full immobility would lead to suffocation. It is hypothesized that they rest by swimming slowly or by finding a location where they can face a strong current, allowing water to be pushed over their gills with minimal effort. Other species, however, are facultative ram ventilators or buccal pumpers, meaning they can actively pump water over their gills by opening and closing their mouths.
Sharks like nurse sharks, which are buccal pumpers, can rest completely still on the seafloor for extended periods. Recent observations of species previously considered obligate ram ventilators, such as the grey reef shark, suggest they may be able to switch to buccal pumping when resting under ledges.