The ocean environment transforms dramatically as daylight recedes. For cetaceans, including whales and dolphins, this transition to darkness fundamentally alters their sensory world. Unlike terrestrial mammals, whales are voluntary breathers, meaning they must remain consciously aware to surface for air. This physiological constraint makes their nocturnal habits complex. When visual cues disappear, these marine giants must adapt their behavior and rely on sophisticated, non-visual senses to navigate, communicate, and survive.
Rest and Sleep Cycles
Whales reconcile their need for rest with the necessity to breathe and remain vigilant through a unique physiological adaptation. This mechanism is called unihemispheric slow-wave sleep (USWS). One half of the brain enters a deep rest state while the other half remains active and alert. This partial consciousness is necessary for the whale to maintain muscle control, regulate buoyancy, and initiate the conscious action of surfacing to breathe.
The active half of the brain allows for constant environmental monitoring, providing vigilance against predators or separation from the social pod. When resting, the eye opposite (contralateral) to the sleeping hemisphere is typically closed. The eye linked to the active hemisphere often remains open, allowing them to effectively “sleep with one eye open” to maintain awareness of their surroundings.
Specific behaviors mark these resting periods, such as “logging,” where a whale floats motionless near the water’s surface, resembling a floating log. Sperm whales are known for resting in short, vertical bursts, sometimes in groups. These brief naps may last only 10 to 15 minutes. Daily rest time varies significantly between species, with some sperm whales spending as little as seven percent of their day sleeping.
Nocturnal Hunting and Foraging
Darkness significantly affects the availability and location of prey, dictating the nocturnal foraging strategies of whales. Many plankton, krill, and small fish engage in Diel Vertical Migration (DVM), rising from the deep ocean to the surface layers as light decreases. This upward movement of prey often makes food resources easier to access for certain baleen whales at night.
Baleen whales (mysticetes), such as Blue and Humpback whales, adjust their feeding dives to follow this migrating biomass. Blue whales may dive to significantly shallower depths at night, staying close to the surface where their primary food source, krill, has migrated. Humpback whales also adjust their dive depth to consistently follow their prey, such as capelin, throughout the night.
Toothed whales (odontocetes) rely on echolocation. For deep-diving hunters like Sperm whales, who pursue giant squid and deep-sea fish below the photic zone, the change from day to night is less relevant to their primary sense. They emit clicks and interpret the returning echoes to form a detailed map of their surroundings and locate prey, a method that functions equally well in total darkness.
Some beaked whales, which are odontocetes, also exhibit nocturnal foraging behavior tied to DVM, hunting prey that comes closer to the surface at night. All whales rely more heavily on hearing and other non-visual senses, like lateral line perception, when visibility drops. Acoustic activity related to foraging in species like beaked whales can show a clear nocturnal cycle, indicating a preference for hunting under the cover of darkness.
Navigating the Dark Ocean
When visual cues are absent, whales depend on sophisticated methods to maintain direction, communicate, and ensure pod cohesion. Acoustic communication becomes the primary means of navigating the dark ocean, with vocalizations like songs, clicks, and calls serving multiple purposes. These sounds travel effectively through water, allowing pod members to maintain contact and communicate over great distances, which is crucial for group coordination.
Whales likely also possess a built-in guidance system independent of light: magnetic navigation, or magnetoreception. Researchers theorize that cetaceans use the Earth’s magnetic field lines, acting as an invisible map and compass, for large-scale migration. The presence of magnetic material in their brains, such as magnetite, may make them sensitive to geomagnetic lines, providing a consistent directional cue regardless of the time of day.
The ocean environment offers subtle, indirect cues that contribute to nocturnal orientation. The presence of bioluminescent organisms in the water can sometimes provide a fleeting visual context or contrast. However, the combination of acoustic memory for familiar routes and the use of the geomagnetic field are the most important factors allowing whales to keep their precise heading through the dark expanse of the open sea.