Is it that you're truly tired and need to rest, or are you just really relaxed and you body is interpenetrating relaxed as sleepy? I often feel really relaxed when I drive, to the point I feel sleepy. I try to listen to upbeat music, something I can sing along to. We often listen to audio books too. If it's a longer drive I like to munch on granola or something similar. Those things help keep me alert.