narcolepsy
"Would frequently get tired in the afternoon or after 15 minutes of driving. Nodded off many times driving, sometimes couldn't remember half the drive. Would fall asleep at work in seminars, at my desk, etc. Always had trouble in college staying awake in class, thought other people were just better at staying interested. 
With Nuvigil, my eyes never get heavy unless I'm really tired. I have much more confidence driving and staying awake at work. Even around family I don't have sleepy spells that make me look disinterested. Amazing stuff with very little side effects. I do often skip a day when I know I won't need it, and I usually feel like a nap after noon. I also take half doses (75mg) and it works almost as well as 150mg."