You control what you day dream about, it's your fantasies. If you are thinking/day dreaming about things that don't "help" you do better in life and get what you "want" (imagining being a movie star, for example), that's pretty much a waste of time? It's okay for a minute or two but if you are doing a lot of day dreaming and most of it isn't useful or gets in the way of your "normal" day (i.e., you aren't paying attention at work or in the classroom, aren't getting your homework or work work done, etc.) then that isn't good?

Here's a good list of articles, some of which might give you ideas how you "change" your daydreaming over so it helps rather than hurts you?

http://www.psychologytoday.com/blog/...er-daydreaming