Why do dolphins sleep with one eye open?

Answer: Half brain stays awake

Saltwater irritates closed eyes — Wrong. Saltwater doesn't irritate dolphin eyes—they're perfectly adapted to ocean water. The open eye is controlled by the awake brain hemisphere during unihemispheric sleep.

Half brain stays awake ✓ — Correct! Dolphins use 'unihemispheric sleep' where one brain hemisphere sleeps while the other stays awake. This allows them to breathe consciously (they can't breathe automatically like us), watch for threats, and maintain pod contact. They switch sides every 2 hours!

Their eye muscles don't fully relax — Wrong. Dolphin eye muscles work perfectly fine! The open eye is controlled by the awake brain hemisphere—it's a neurological phenomenon, not a muscle problem. They can close both eyes when fully awake.