WHY does a dolphin sleep with one eye open?

 Dolphins have a remarkable ability to sleep by selectively shutting down one hemisphere of their brain at a time. While one hemisphere rests, the other remains awake and alert. To ensure constant vigilance, dolphins keep one eye open to monitor their surroundings. When the right hemisphere sleeps, the left hemisphere takes charge, resulting in the right eye remaining open. Similarly, if the left hemisphere sleeps, the right eye stays vigilant. This unique sleep pattern allows dolphins to maintain awareness of any environmental changes while still getting the rest they need.