I asked a while ago whether anyone else had sudden tiredness and a few responded likewise, so this update is for you and anyone else affected.
Every now and then I have to put my head down and go to sleep for no apparent reason, completely knocks me out. It's not related to any regular food and initially I thought I had a thyroid problem.
I've been on at my doc for ages, who finally referred me to an endocrinologist. When I saw the endo he suggested a glucose tolerance test 'because he sees lots of coeliacs who are affected by reactive hypoglycemia'.
I had the glucose test yesterday, and 40 mins after drinking the (sickly) glucose drink I fell asleep, curled in a ball in the waiting room. The nurses were shocked by the effect it had on me and insisted on monitoring me for the rest of the 4 hour blood test.
If you're querying sudden tiredness or ongoing fatigue please consider reactive hypoglycemia as a cause.