You might be affected by Delayed Sleep Phase Syndrome. Delayed Sleep Phase Syndrome (DSPS) is a sleep disorder in which an individual finds it difficult to fall asleep late in the night, and sleeps well into the afternoon if not awakened.
This graph shows a DSPS-person sleep hours (in blue) and the shift per day:
I recommend reading this article from supermemo.com, and downloading a free sleep phase log software that can help you find out what is going on with your sleep rhythms.
You might also want to check out this Google Tech Talk about Nutrients (and sleep) for better mental performance.
I also recommend this page about sleep disorders in general.