Getting sleepy while driving is a common occurrence. When I was young I fell asleep driving and hit a house at 35 mph. The resulting injuries made me pull off road safely (like in a rest stop on freeway) lock all doors a lay down and sleep for an hour or two ever after. Driving on open road is like being in a comfortable bed gently moving thru the night. Best to open window(s) and get natural fresh air to stay awake. If still drowsy continue to pull over and take nap as you are doing. Never fall asleep at wheel; you are a danger to yourself and others.
You are either sleeping way too few hours (for example if you get 4 hours a night), or you could have a sleep disorder like apnea. Definitely tell your primary so this doesn't happen any more.