If you're getting drowsy on a regular basis in the same vehicle, check your exhaust system for any leaks. Carbon Monoxide is an oderless, colorless, tasteless gas that's a by-product of the emmisions of your car. It bonds faster to your hemoglobin than oxygen, so the oxygen, even if present, doesn't get into your bloodstream and to your brain. I'm a truck driver, and one of my previous employers had a truck that would just put you to sleep if you had the windows up. Everyone who drove it felt the same way, and we could never find the leak. Still, there was almost no doubt that CO was leaking in the cab somehow.
Even if there is no leaks, the airflow around your vehicle, with the differing pressures because of that airflow, may cause CO to be drawn into the passenger compartment. Again, you can smell exhaust, but not CO and you won't notice it. You just feel sleepier and sleepier.
I driven for one of the FED-EX companies for over the last 6 years. I drive predominately at night. The key is to know your body and recognize the signs. If you start falling asleep, pull over in a rest stop or such and catch a 15min "power-nap". Coffee, etc, will help, but stay away from drugs as much as possible (like no-doze). They'll keep you awake, but your reflexes and judgment will be worse even though you feel like you're responding better. Also it will make it harder to catch a nap when you need to.
Each body is different in the amount of rest it needs. If you get sleepy, sleep. It's simple as that. It may not only be the lack of sleep, but the lack of quality sleep. Many people sleep really poorly, but never know it, and feel run down quickly. There may be other health problems, like diabeties, heart condition, etc that can affect both your sleep and your ability to stay awake. Above all, pay attention to what your body's telling you.
Ross