Well, generally speaking, daydreaming is not bad for the normal eyes. As we know that the default condition of the normal eye is to be relaxed. So if you have normal eyes, it will maintain relaxed eyes, even when you are under adverse conditions, such as day dreaming. But on other hand, it can be bad for your eyes, when you have myopic eye, for daydreaming badly affect the myopic eye. and it may lead to eye strain. And you can try singing or listening to music with a good story that gets the mind thinking of the characters in the mind. In this way, it will relax the mind, eye muscles and bring clear vision. Anyway, it will be beneficial to your eyes.