I think I know why I was tired yesterday. Apparently I am borderline anaemic. I found this out today when I tried to give blood but my haemoglobin levels were too low. (Not just 'You can't donate today' low, but 'We're going to send your blood off for tests and then you should go see your doctor' low.)
I blame work. I've never had a problem donating before.
Speaking of work, I'm 11 till 7 tomorrow. Oh, the joys of a new catalogue.