I'll just add a note about this because someone in my family has it:
http://en.wikipedia.org/wiki/Circadi...sleep_disorder
It's totally different to having a temporary change in your sleep cycle though which you will be able to reset and adjust back when your lifestyle changes.
I bet there are a lot of night owls out there who have this tho and have never heard of it so thought I would put it up out of interest
