Dolphins are highly intelligent, social marine mammals whose lives are fundamentally tied to the demands of a fully aquatic environment. Unlike terrestrial animals, their need to consciously control breathing and remain vigilant against predators means their nighttime behavior is quite different. Their nocturnal activities are a complex balance between the need for rest, the search for food, and the requirement for safety in the ocean.
How Dolphins Rest
The concept of “sleep” for a dolphin centers on a process called unihemispheric slow-wave sleep (USWS). This is a remarkable adaptation where only one hemisphere of the brain rests at a time, entering a deep, slow-wave sleep state. The opposite half of the brain remains awake and alert, maintaining the consciousness necessary for survival.
This partial alertness is necessary for two primary reasons: to manage breathing and to watch for threats. Since dolphin breathing is a conscious, voluntary act, the resting half of the brain would otherwise cause the animal to stop breathing and potentially drown. The awake hemisphere ensures the dolphin surfaces regularly to take a breath through its blowhole.
During USWS, the eye opposite the sleeping brain hemisphere is typically closed, while the other eye remains open to scan the environment. Dolphins often rest by swimming slowly and steadily, maintaining a gradual pace in tandem with their pod, or they may simply “log,” which involves hanging motionless near the surface of the water. Each brain hemisphere cycles through rest for several hours, allowing the dolphin to achieve the restorative benefits of sleep while never fully losing awareness.
Habitat Use After Dark
A dolphin’s nighttime location is determined by its species, population, and environmental factors. Coastal bottlenose dolphins frequently move into shallower, more protected waters like bays or estuaries when resting. These areas offer a slight reprieve from strong currents and fewer large predators.
Oceanic species, living far from shore, cannot seek shallow water safety. Instead, they maintain slow, steady movement near the surface in deeper water, allowing them to rest while continuing to breathe without interruption. For some populations, like the spinner dolphin, a distinct pattern is observed: they spend the daytime resting in sheltered bays and move offshore to feed at night.
A dolphin resting area is generally less defined than a terrestrial animal’s den but provides relative safety and reduced energy expenditure. For coastal populations, this movement often involves utilizing habitats near inlets or river mouths. Telemetry studies have shown that some coastal dolphins will even travel up freshwater rivers for short distances during the night.
Nocturnal Foraging and Predator Avoidance
Nighttime is not exclusively reserved for rest, as many species use the cover of darkness for active hunting. This is true for dolphins that prey on species undergoing diel vertical migration, such as squid and fish, which move closer to the surface at night. These hunters forage intensively, taking advantage of concentrated prey availability.
Vigilance against predators is constant during all nocturnal activity. Large sharks, such as tiger sharks and bull sharks, are often most active at night, increasing the risk of attack. Dolphins may change their behavior, sometimes avoiding shallow foraging grounds when large sharks are present.
To counteract this threat, dolphins rely on their social structure and advanced sensory abilities. Pods remain grouped together, providing safety in numbers and a collective defense system. Their sophisticated echolocation remains fully operational, even while one half of the brain is resting, allowing them to detect potential threats in low-light conditions.