How do marine animals sleep?

Sperm whale sleeping. (Credit: iStock photos)

Rock lobsters, for example, are nocturnal and conserve energy by remaining motionless for up to 14 hours a day—often under cover of a dark overhang to give their lidless eyes a break from sunlight. Clams and other bivalves rest by relaxing the adductor muscles that hold their shells closed. This action allows them to open wider and increase their respiration. Alternatively, sessile (stationary) animals fixed to the seafloor, such as corals, rely more heavily on cues from temperature and sunlight to reduce or increase their metabolic activities throughout the day—an internal clock we humans also use called a circadian rhythm—or cues from tides (circatidal rhythms).

Other marine animals take a load off by turning a lobe off. Many species of shark as well as cetaceans (dolphins and whales) will quite literally shut down half of their brain to rest, a process known as unihemispheric slow-wave sleep (USWS). Like us, dolphins and whales are mammals that need to breathe air. In USWS, these animals can “sleep swim” their way to the surface for a breath. Sperm whales famously sleep in a vertical orientation, like floating pillars, during USWS. Obligate ram ventilators—species of shark that need to pass water continuously over their gills to breathe—also rely on USWS to breathe during sleep by swimming on autopilot. Great white, mako, and tiger sharks have all been known to enter currents to minimize their movement and increase water flow over the gills during USWS—a resourceful behavior known as dynamic surfing.

Finally, there are some species that can both rest and sleep. Parrotfish sleep by reducing activity in both lobes of their brain to allow for deep, restorative rest similar to what we humans experience. Night divers regularly find these fish cocooned in a clear mucus bubble—an ingenious trip alarm that parrotfish secrete to evade late-night intruders.