Jetlag
My wife can't believe how easily I can fall asleep in most places to catch a little nap when needed. Usually just the shutting of the airplane door is enough to bring on a few zzz's. Strangely though, while true with any flight, whether large or small, smooth or rough, in the US, as soon as I get on a international flight I lose all ability to sleep whatsoever.
So, it's 6am in Belgium and I'm waking up, with a ripping case of jetlag, from 11 hours of sleep caused by a 24hour period of non-sleep getting here and spending a half day with a customer.
Perhaps I'll figure it out on the way home. :)