Melatonin?

I believe I may be Bipolar, perhaps cyclothymic. I have not seen a doctor yet but I will be doing this very soon. I have a very strong case of insomnia, as I am completely unable to sleep at night naturally unless I have been awake for at least 20 hours (it can go as long as 40+...I have never seen nor heard of such inhuman sleeping patterns, and to top it off it's in myself). Anyways...

I have been taking "Nature Made" melatonin in order to sleep at night. It works very well, as I have been sleeping better than ever before, and I actually am able to get up in the morning as well - a problem I've always had in the past. It seems I'm getting some quality sleep as well. However...

The bottle explicitly states that you should not take melatonin if you have an autoimmune condition, DEPRESSIVE or endocrine disorder. Bipolar disorder certainly constitutes depressive. Ever since I have been taking it, I have noticed some strange changes in myself every day. I am not sure about the explanation for this, as I have been taking it at night, but ever since I have been taking melatonin, my mood has been unusual. Usually I am high, with occasional cycles of depression or "mixed" and sometimes normal. However, I have not been experiencing ANY high periods, and I have been more depressed lately. Mostly mixed, and rarely normal, but never high and usually more depressed than anything else. It could be purely coincidental, or it could be that I am more depressed as a result of being more high previously.

I think it's definitely a good idea to try to not take melatonin for a while to see what happens. I know I'm going to have some trouble sleeping and the biggest problem is that when I'm not taking melatonin, I can't go to bed ANY EARLIER than 3 AM (and even then I'd be lucky to fall asleep at that time) and unfortunately I have class early in the morning. I really hope it doesn't interfere with this, but hopefully I've gotten my sleeping schedule properly set at this point.

Anyway, can anyone here explain to me how melatonin and bipolar disorder mix together? I do realize that I have not been diagnosed yet, but this is not the issue at the moment.