Dolphins Sleep With Half Their Brain at a Time
One brain hemisphere sleeps while the other stays alert — keeping one eye literally open.
Dolphins practice unihemispheric slow-wave sleep (USWS) — one hemisphere of the brain sleeps while the other stays awake and alert. This allows them to continue swimming, watch for predators, and surface to breathe while still getting rest. The eye connected to the sleeping hemisphere closes; the other stays open.