narcolepsy
"Provigil works great for me, at 200 mg once in the morning. I usually feel dull-witted and fatigued, but I don't suddenly "switch off". After about a week of steady doses, however, I often develop a bad headache and have to quit taking the stuff for a few days. Maybe that's a good thing, if it keeps me from getting habituated. Ritalin did nothing except make my hands shake."