I think it's very important to have a hobby, or to have something that will keep your mind occupied. If you're working hard at work or school, you've got something you're working towards or for. You're doing something purposeful. If you have a hobby that you're passionate about then you push yourself to do well in it, maybe even to be the best at it. Both of these things allow your mind to focus on something, or maybe take your mind off other things. Without a focus or a purposeful subject in your life, I find that it becomes difficult to concentrate. You feel as if you're just floating through life, getting nothing done, getting nothing out of life. I, personally, need to feel as if I'm doing something with a goal. If I'm working towards something and enjoying it, feeling satisfied by it, then I feel as if I'm making some of the best of my life. I don't feel as if I'm wasting my time away doing nothing.
Keep your mind occupied with things that you enjoy, or work so that you will get to those things that you enjoy. Just make sure that you are always enjoying it, because otherwise it loses its objective.