Why am I awake at night and tired exhausted during the day?

I just don't get it and it's so frustrating. I feel so alone with this problem. When the sun comes up I get so exhausted and can sleep soundly. Basic functioning during the day is like scaling Mt. Everest. At night I get so much energy and am most awake around 2-4 am. Around 7 am is when I start to get tired. I'm a graduate student so I have to do my homework late at night and needless to say I miss a lot of classes. Rx sleeping pills leave me extremely tired during the day and are minimally effective at night. Even 5 mg of melatonin doesn't work for me. So, what's wrong with my circadian rhythm and why am I so weird? Working third shift during a 6 month gap between undergrad and grad was amazing and felt natural, but it was hard to go go doctor appointments and participate in life as I snoozed when everyone else was awake. Is there anything I can do to help my sick brain? Thanks!