They sleep with only half their brain at a time, keeping the other half alert for predators.
They sleep during the day with their eyes open, which helps them to stay alert and ready to respond to any danger approaching.
Penguins often sleep with their eyes half open which is a strategy to remain aware of potential threats.
Most fish sleep with their eyes open, as they don't have eyelids.
They don't have eyelids either, so they sleep with their eyes open.
They sleep with one eye open which allows them to stay alert for any threat.
Some frog species sleep with their eyes open, but their brains are in a state of reduced activity.
They sleep while standing with their eyes open so that they can stay safe.