SLEEPING WITH ONE EYE CLOSED

I go to sleep with half a brain.

This may sound strange, but I’ll explain.

I live in water and I find

I always need some of my mind.

So, one eye’s closed to let me keep,

part of my brain in restful sleep.

The other’s open to watch out,

in case a hungry shark’s about.

Plus, I must still get air at rest.

to fill the lungs inside my chest.

So, I’ll swim near the surface where

my wake half moves me up for air.

Once half my brain has rested, I’d

start snoozing on the other side.

Then my eyes switch, so I’ll still see

Whatever’s swimming close to me.

What am I?