Jeffrey,

I get the same problem where several times a day, I fall asleep randomly when I doing something - fortunately only when I am sat down. I have even fallen asleep half way through writing and email!

This is mainly after something has overtaxed and much like a PC I need a reboot. Normally this clears my head and I feel much better. However, the down side is that I always find my head slumped forward with my head on my chest which ins't good for my neck