What should I do when I'm tired, but do not have the time to go to sleep?

Make the time! Sleep is the body and mind's time to rejuvenate, and the importance of good quality sleep cannot be overstated. Sleep deprivation can lead to all sorts of problems, not to mention how unsafe it is to drive while sleepy. Being tired is your body's way of saying you need sleep - so listen!
Sleep. Try getting sunlight exposure, taking a walk, exercise, changing activities to something more stimulating, and making sure you don't lay in bed during the daytime!