Dolphins sleep in single-hemisphere sleep, which means that dolphins use only half of their brains to rest at a time. One side of the dolphin’s brain is always awake, allowing the other side to go into deep sleep.
Dolphins sleep near the surface of the water, and they do so motionlessly and with one eye open. They remain near the surface of the water because they must periodically return to the open air in order to breathe. Moreover, sleeping dolphins keep one eye open in order to search for predators.
This means that the dolphin never actually sleeps because the dolphin’s brain must always be active enough to start breathing. When they sleep, the dolphins let one side of their brain doze off while the other half paid attention.
When it comes time to rest, dolphins close only one hemisphere of their brain and the other eye (the left eye closes when the right hemisphere is asleep, and vice versa). Like whales, dolphins and related Dara porpoises sleep with one eye open so that half of their brains are asleep. In this sleep mode, whales and dolphins do not go into deep sleep, with only half of their brains unconscious and the other half awake.
What Happens When Dolphins sleep
When they sleep or rest, one hemisphere of their brain remains stationary, while the other shows signs of activity and stays awake. Dolphins and whales do not drown even when half of their brains remain still and maintain a stable posture during sleep. In fact, the animals were all asleep, but unlike humans, their minds didn’t achieve the same sensory cessation and low-consciousness.
To continue breathing while sleeping, whales and dolphins must control their stomata, the skin that dolphins and whales voluntarily control. One of the reasons marine mammals need to keep half their brain active while they sleep is that their breathing must always be consciously controlled underwater, whereas human breathing is involuntary. These mechanisms are part of the underwater response of marine mammals, adaptations to life in the aquatic environment, and contribute to the sleep process.
How Dolphins Facilitate Their Sleeping
Dolphins and whales can selectively use their organs, which helps them breathe longer underwater and maintain sleep patterns in one hemisphere. One-brain sleep also allows dolphins and whales to monitor their environment while they sleep, looking for predators or other threats. Second, slow-wave hemisphere sleep allows dolphins to keep an eye out for danger as they rest.
Typically, dolphins sleep with only one hemisphere of their brains at a time during slow-wave sleep, maintaining sufficient awareness to breathe and to be on the lookout for possible predators and other threats. Individual dolphins also go into deeper sleep, mostly at night.
Brain waves from sleeping captive dolphins (Tursiops truncatus) showed that one side of the dolphin’s brain was “awake” while the other was in a deep sleep called non-REM sleep. Observations of captive and wild bottlenose dolphins suggest that they sleep in two ways: resting quietly in the water vertically or horizontally, or swimming slowly with another aquatic animal.
How Dolphins Sleep While Needing to Breath
Dolphins sleep by resting on the surface of the water and breathing frequently, or by swimming slowly and steadily near the surface. Dolphins can sleep on the bottom in shallow water, regularly rising to the surface to breathe. Non-hemispheric sleep allows the dolphin to remain alert enough to reach the surface of the water to replenish its lungs with oxygen and not drown. When sharks sleep, they handle the current, allowing them to rest while the moving water continues to supply them with oxygen.
Dolphins usually sleep at night, but only for a few hours at a time; they are often active late at night, perhaps combining this alert period with feeding on fish or squid before rising from the depths. From a distance, sleeping whales and dolphins look like logs in the water; sailors should stay away from resting whales and other marine mammals, as well as during the journey of these animals.
Do Whales and Dolphins Really Sleep?
The shortest answer to this question is yes, whales and dolphins sleep, but it’s more of a rest because their brains aren’t completely shut down, which is very different from how humans and other land mammals sleep. Research has shown that even during “dolphin sleep”, dolphins can perform the same activities uninterrupted for long periods of time.
During this time, the other half of the brain remains engaged at a low alert level, observing its surroundings, watching for predators, and reminding the dolphin or whale to come up to the surface every few hours to take an important breath. . The average dolphin breathes nearly 4-5 times per minute and can hold its breath for up to eight minutes while sleeping or resting. When a dolphin breathes, air enters directly into its lungs, expiration and inhalation rarely take more than a fraction of a second and take on average about 2-3 breaths per minute (Ponganis et al, 2003).
Dolphins need about eight hours of sleep each day, but their brains sleep in shifts, unlike ours. Sphereless sleep was thought to have evolved because of the need for dolphins to breathe on the surface of the water, but it may also be necessary for defense against predators, the need for toothed whales to stay in their tight flocks, and to regulate their internal bodies.
