Baby only opening one eye at a time?

m231

My DS does this occasionally.  It does it with both eyes though.  He also can open both at the same time.  But every once in awhile, he'll only open one of his eyes.  Then he'll do it with the other eye.  We call it his "pirate eye".  Should I be concerned or is this normal?  I know since he opens both eyes at the same time that it's probably not a big deal.