What can cause falling asleep while driving for no reason?

Lots of things: Inappropriate daytime sleepiness can be seen with lots of things including:occasional insomnia, effects of prescription or otcmeds, sinus related issues, hormonal issues, lack of food, low blood sugar, etc. Sleep apnea (disturbance of sleep with episodes of low oxygen) commonly has daytime somnolence, unrefreshed awakening. See your doc.