How often can you break your sleeping pattern without it affecting you too much?

I have sort of the same problem as you; but I eventually figured out that I can usually only sleep after I've been awake for 14 hours. I would suggest an alarm clock or a subconcious message in your mind (sounds weird but just before I fall asleep, I think to myself what time I need to get up and my mind keeps track of time when asleep) so get up around 8am or 7am every day for a while to get used to getting up earlier. This makes you wanna go to sleep sooner - thus changing your sleeping pattern :)