During the night, when they sleep, dolphins swim around slowly in a counter clockwise direction. Individuals are spaced in tight schools, no more than two body diameters away from neighbouring dolphins in their group. And when an entire group of Pacific white- sided dolphins is half asleep the open eye of each dolphin is directed at the other group members, towards the centre of the swimming group. Swimming with one-eye open and facing the group enables dolphins to maintain a cohesive, integrated and tight formation. As well as keeping a close eye on other group members the dolphins can scan the environment past their schoolmates looking for any signs of danger.
Dolphins sporadically switch their positions within the group when half-asleep swimming.
They also change the open eye, and alter the sleeping hemisphere, to ensure that they always have one eye on their schoolmates. The other half of the brain, the one that's been on watch, can rest when they switch sleeping hemispheres. Thus dolphins can fully rest their brains during their sleeping session while still being able to breathe and swim voluntarily. Changing positions gives dolphins a short break in formation, which may help school members to inspect their immediate vicinity for predators such as killer whales, at a time when they are most vulnerable. They remain silent when swimming in this half-asleep formation.