Narcolepsy is a disease, a problem in the brain. Narcoleptics fall asleep instantly with no warning, not feeling tired or like they're about to nod off. Falling asleep at the wheel, like falling asleep in front of the TV, is different.
I have a problem staying awake sometimes, now that I'm getting 'up there'. I do a lot of long-distance driving, and I know the risks. I keep a few 5-hr Energy drinks in the glove compartment, and if that doesn't do it (sometimes it doesn't) I'll pull over for a nap for an hour or so.
It might be inherited, I don't know. But at least in my case I think it's just age. When I was younger, I often had a hard time falling asleep. That's not a problem anymore. Now I have trouble staying awake!