ive been playing with my eyes all my life and never had a headache
i did ask my optometrist about it once, he said its not bad for your eyes at all. its just relaxing the muscles. i used to amaze my peers at school by pointing out what was in the magic eye books as soon as they opened the page. theyd try all sorts of things like moving it back and forth and looking past it and sliding it up into their vision. good times