How do dolphins sleep without drowning? Is it true that they sleep with one eye open?

Dolphins, being air-breathing mammals, have to resurface every now and then to breathe fresh air. So, one wonders when and how they manage to get sleep.

Dolphins are a small group of whales and like other whales they have lungs instead of gills. They breathe through a single nostril called the blowhole located on top of the head. The blowhole is opened during their frequent trips to the surface to expel and inhale air.

Even though respiration has to continue with not more than few minutes’ respite between breaths, dolphins sleep an average of eight hours a day, but only allow one half of their brain to doze off at a time. They cannot ‘switch off’ both hemispheres of their brain and have deep sleep underwater, or they would drown. So sophisticated is the dolphin’s sleep system that it naps at the surface of the water, with half of its brain alert and the opposite eye open, while the other half of the brain is asleep with the opposite eye closed. (See photo above.) During sleep, the dolphin alternates position to allow each half of the brain to get its turn. In effect, the dolphin is awake and asleep at the simultaneously.

Not only do dolphins swim near the surface of the water but sometimes they swim vertically or horizontally as well. This phenomenon is known as logging. In other cases, dolphins might also sleep while slowly swimming next to another animal. Typically, in the case of dolphins, they eat, sleep and rest while their mother is swimming, obviously for protection. The mother uses her slipstream to tow them along, and she during the process might sleep as well.

Dolphins do not go into a deep sleep. Instead, they nap for 15 to 20 minutes until they need to come up to breathe. Even though dolphins remain half-awake while napping, it is not the case with a newborn, since a newborn does not have enough blubber and fat which helps an adult dolphin to keep afloat, it is imperative that the mother must not stop swimming. Otherwise, the calf will drown.

Additionally, dolphins are voluntary breathers, unlike us. It means that they have to inform their body when they need to breathe, and that is not possible if they are not focusing on their breathing, which is also the same as meditating. If dolphins were to be involuntary breathers like us and their brain was programmed to force the animal to breathe through its blowhole automatically, then the chances are that it would end up drowning, keeping a chase of its breaths. Therefore, dolphins will only breathe when they know that they are, and especially the blowhole is above the surface.