Nodding off or feeling drowsy while driving is extremely dangerous. Common reasons include the monotony of the open road, particularly straight roadways with nothing to look at (I'm thinking of Interstate 5 in CA!). Also, lack of fresh air. Be sure your car does not have a minor exhaust leak. This would allow CO2 to enter the vehicle, making you sleepy.
Yes monotony can be a problem. Did you eat too much? Drink water, soda, coffee....something- no alcohol before or during driving. If cold out don't have the heat on too high for too long as that can woo you to sleep. Open the window to have cold air in your face. Change the music. Talk to someone on your cell.