How do you sleep when you’re in a three-dimensional space like the ocean? To complicate this, breathing for killer whales is not an automatic function, as it is for us. Being mammals, they can only breathe at the surface, and must consciously open their blowhole to exhale.
Killer whales overcome this by shutting down half of their brain at a time! Scientists call this unihemispheric sleep, and this allows them to have the benefits of sleep, while still maintaining breathing, body temperature (thermoregulation), and awareness of their surroundings.
During such times they may be seen floating at the surface, a behavior called logging. Family groups sometimes will log together, this is known as a resting line.