How do orcas sleep?

 Orcas sleep in a very different way to humans. We have a breathing reflex and when we sleep, we continue to breathe automatically. Orcas cannot sleep in this way, they have to keep conscious, even when they are sleeping. This is because their breathing is not automatic, they have to decide when to breathe, and they must be conscious even when sleeping. If orcas went into a deep unconscious sleep, they would stop breathing and suffocate.

Orcas only allow one half of their brains to sleep at a time; the other half stays alert enabling them to continue breathing while they are looking out for dangers in the environment. They only close one eye when they sleep; the left eye will be closed when the right half of the brain sleeps, and vice versa. This type of sleep is known as unihemispheric sleep as only one brain hemisphere sleeps at a time. Orcas periodically alternate which side is sleeping so in this way they can get the rest they need. When sleeping, orcas swim very slowly, close to the surface.