Can't stay awake while reading?

What would cause someone to fall asleep anytime they try to read something? No matter how much they want to read something, or know they have to read so many chapters for a quiz. I had this problem for about 6 years, it came out of nowhere and it went away one day out of the blue. I tried during up while reading, standing, bright lights, different times of day, taking breaks. I used to love to read, but when this problem began, I started hating to read. No matter how hard I tried, I couldn't stay awake. I'd start yawning after about one paragraph. The worst experience I had with this condition was when I was taking a national exam in high school and fell asleep reading a word problem. Anyone ever go through this?