I did a bit of figuring out today and I think I have a 26 hour rhythm. If I don't have to get up at a specific time I can quite happily sleep for 10 hours, then spend the next 15-16 hours awake. Forcing myself awake at 7am every day is forcong myself into a false rhythm that doesn't suit me. No wonder I'm tired all the time.
Either that, or I have Seasonally Affected Disorder brought on by the short days.
Dunno. It's a worry.