Ride Safe and Alert!
At a certain point in ones' motorcycling career, after a heavy meal or just about any time in Kansas or Nebraska, the drowsies are going to set in. We sleep approximately 220,000 hours in a lifetime (230,000 if you make it a habit of getting out of your sleeping bag at noon at every BMW rally), and none of them need to be on the highway.
The most important thing to remember is that caffeine is your enemy. It does stimulate the nervous system, but drink too much coffee and the result can be that your brain falls asleep while your body is still awake, and you experience what are known as "Hyponagogic hallucinations." As enjoyable and inexpensive as such hallucinations are, they're a sign that you need sleep. Yawning is a more common sign.
Avoid eggs, meat, or other foods that contain high amounts of tryptophan, which also can cause slowing of the thought processes. For some ginger, ginseng, or licorice can provide a mild natural stimulus. Beemer-bago riders (those with radios) can tune in Gangsta Rap, Polkas, and Sousa marches which are also good for perking yourself up - or for attracting the local deer out of their hideaways and into your pathway.
Really, the best thing you can do is pull over and take a nap if only for ten or twenty minutes. A short brisk walk, a series of stretches, or yelling at passing cars may also awaken you. However, yelling at passing police cars is not recommended and may result in your need to make bail.
So, next time you feel drowsy, get off the motorcycle take a short nap or a brisk walk, or at least practice stretching. The life you save may be your own and you only have one of them.