How to Determine if You Have Insomnia or Different Sleep Patterns

How do I know if I have insomnia or if I just have different sleep patterns? When I'm left to set my own schedule, I usually sleep from 10AM to 6 or 7 PM, and do my daily tasks when I'm awake, and I feel super rested and productive. I get so much done and feel amazing! However, trying to sleep from 11PM or midnight to 8 or 9 AM leaves me feeling exhausted and unproductive. I've tried really hard to keep to this sleep schedule for a few months now because I need to for school, but I don't feel productive and am getting nothing done; I feel so much more awake at night and get more done, so it's rough when I have to force myself to stop at midnight to sleep. Is there something wrong with me, or is it possible I'm just wired this way? Should I possibly try sleeping when I get home from school and then just doing my homework in the early morning before class? I don't know if that's a good idea either though since I don't get home at a set time each day.