How Do Dolphins Sleep While Swimming?

They sleep with one half of their brain at a time. When a human enters deep, slow-wave sleep, electrical activity normally spreads across both cerebral hemispheres. But the brain activity of dolphins is quite different. One hemisphere can produce the large, slow electrical waves associated with sleep, while the other shows activity closer to wakefulness. Later, the two sides exchange roles. This is called unihemispheric slow-wave sleep. It is often described by saying that half of the dolphin's brain switches off, while the other half remains awake. That is a useful simplification, but the sleeping side is not inactive. And the waking side is not necessarily behaving exactly as it would during active hunting or play. Instead, the dolphin enters a mixed state, part sleeping, part awake, with enough coordination preserved to continue functioning in the water. A resting dolphin may swim slowly in a steady direction, circle repeatedly or remain almost motionless near the surface. It can continue controlling its posture and periodically rise to breathe without both hemispheres entering ordinary wakefulness. Electrophysiological studies have recorded breathing during uninterrupted unihemispheric sleep. The dolphin can surface and breathe without the sleeping hemisphere suddenly returning to a fully wakeful pattern. So although dolphin breathing is under unusually strong behavioral control, the common claim that a dolphin must consciously remember to breathe every few minutes is too simple. The dolphin's nervous system can coordinate breathing with sleep, but only because its form of sleep is radically different from ours. Dolphins may rest with one eye closed and the other open. Studies recording both brain activity and eye position have found a relationship between the sleeping hemisphere and the state of the eyes. Usually, the eye connected most directly to the more wakeful hemisphere remains open while the other eye closes. The relationship is not perfect in every moment, but it allows the dolphin to preserve some visual contact with its surroundings while part of its brain sleeps. This is sometimes presented as a simple predator alarm. One eye sleeps while the other watches for sharks. Predator detection may certainly be one advantage, but the evidence is more complicated. In one study of resting Pacific white-sided dolphins, the open eye was more often directed towards other dolphins than towards the outside of the group. That suggests the animals were using their open eye to maintain contact and formation with their companions. However, those dolphins were living in an enclosure where there was no immediate predation risk. In the wild, the balance between watching companions and watching for danger might be different. Vision is only part of the story. Underwater, visibility can be poor, especially at night or in cloudy coastal water. Dolphins therefore monitor their surroundings using sound. They produce rapid clicks and analyze the returning echoes, allowing them to detect and identify objects through echolocation. In one remarkable experiment, two bottlenose dolphins were trained to search for simulated targets using echolocation. They repeatedly scanned their enclosure and reported when they detected a target. Both dolphins performed the task continuously during five-day sessions. One female was then tested for 15 consecutive days and maintained extremely high performance without an obvious decline. This does not prove that she was echolocating while one particular hemisphere slept. The researchers were not simultaneously recording her brain activity, so they could not identify precisely when she was sleeping or which hemisphere was producing slow waves. But the experiment demonstrated that dolphins can maintain a level of environmental monitoring that would be impossible for a severely sleep-deprived human. Unihemispheric sleep provides a plausible way of combining that vigilance with the brain's need for rest. A dolphin can reduce activity in one hemisphere while preserving enough sensory processing and behavioral control to detect obstacles, follow other dolphins, and perhaps notice an approaching predator. Then the two sides can exchange roles. Not every sleeping dolphin must swim quickly. Some rest almost motionless at the surface, while others swim slowly and steadily. In shallow environments, dolphins have also been observed resting near the bottom and periodically returning to the surface. But remaining completely unconscious and immobile would create several problems. The dolphin could sink too deeply, drift away from its group, or lose the posture required for breathing. The open ocean also offers no den, nest, or tree in which it can hide. Even a large dolphin may face attacks from sharks, and young animals are especially vulnerable. Continuous or slow movement can therefore preserve breathing, group cohesion, and environmental awareness at the same time. Movement may also help with temperature regulation. Water removes heat from the body far more effectively than air. Keeping part of the nervous and muscular systems active could help dolphins maintain body temperature while the other hemisphere rests. Although breathing and vigilance are usually considered more immediate explanations, unihemispheric sleep is therefore not a solution to one isolated problem. It allows several essential functions to continue together. For a newborn calf, sleep presents an even greater challenge. A young dolphin must remain close to its mother, surface more frequently than an adult, and learn to coordinate swimming and breathing almost immediately after birth. The calf often swims beside its mother in a position where her movement helps pull it forward through the water. Studies of dolphin mothers and calves found very little typical surface rest behavior during the early period after birth. In one study of three bottlenose dolphin mother-calf pairs, surface resting disappeared for at least the first two months, while the animals spent almost all the observed time swimming together. Other observations of bottlenose dolphins and killer whales also found that mothers and newborns remained unusually active during the first weeks after birth, with resting behavior increasing gradually over the following months. That does not necessarily mean that they received no sleep whatsoever. Slow coordinated swimming may conceal periods of unihemispheric sleep that cannot be identified from movement alone. But it does show how different cetacean sleep is from the long, obvious periods of immobility seen in many newborn land mammals. For a dolphin calf, continuous movement may help its surface remain beside its mother and reduce the risk of separation or attack. There is one final difference between dolphin sleep and our own. Humans alternate between several sleep stages, including rapid eye movement, REM sleep. During REM sleep, most skeletal muscles become strongly relaxed, while the brain becomes highly active. This is the stage most closely associated with vivid dreaming. Such deep muscular relaxation would be hazardous for an animal that must maintain its position in the water and continue surfacing. Convincing REM sleep has not been clearly demonstrated in dolphins. Researchers cannot yet say with certainty that it is entirely absent, but if dolphins experience anything resembling REM sleep, it must be extremely reduced, unusually brief or expressed differently from the familiar form seen in terrestrial mammals. That also means we do not know whether dolphins dream or what a dolphin dream would be like. Their sleep cannot simply be treated as human sleep, divided neatly between two halves. It is a specialised state, shaped by the demands of living permanently at sea. A sleeping dolphin is not fully unconscious. One hemisphere enters slow-wave sleep, while the other remains more wakeful. The dolphin can continue swimming, maintain its posture, monitor its surroundings and rise to breathe. Later, the two sides exchange roles. This does not make the dolphin permanently alert or remove its need for sleep. Sleeping with half the brain at a time is what makes sleep possible at all.