Curiosity 4 of 10
4. Dolphins sleep with half of their brain at a time
Dolphins don't sleep the way we do. They rest one half of the brain at a time, while the other half stays awake, and they often keep one eye open during sleep.
Why dolphins can't switch off completely
For us, breathing is automatic, even during sleep. For a dolphin, it isn't: it has to rise to the surface and breathe deliberately, through the blowhole on top of its head. If it slept fully, like we do, it would risk not coming back up to breathe.
The solution is so-called unihemispheric sleep. One half of the brain goes into deep sleep while the other handles breathing, movement and attention to the surroundings. After a while, the two halves swap roles. The eye that stays open is the one on the opposite side of the awake half.
This way of sleeping also helps keep the group together and spot predators nearby. Dolphins have been observed resting while swimming slowly or floating near the surface, always with part of the brain on alert.
Dolphins also recognize each other by sound. Each bottlenose dolphin develops its own whistle, which works as a kind of name, and other dolphins can imitate that whistle to call a specific companion.
Did you know? Dolphins aren't the only ones. Some birds, such as ducks, can also sleep with half of their brain. When they are at the edge of the group, they keep the eye facing outward open, toward where danger may come from.
