This link describes research showing that 1 in 24 people report driving while drowsy. Insufficient sleep and sleep disorders like obstructive sleep apnea are to blame. Drowsy driving was more common in young and older drivers. I imagine the younger ones were sleepy from too little sleep and the older ones from sleep disorders and perhaps sedating medications.
This study highlights the importance of safe driving. Don't drive if you feel drowsy. If you get sleepy while driving, pull over and take a brief nap. Or if your traveling with another person, switch drivers.