When you yawn or feel drained go to bed, take a nap, or find a quiet toilet cubicle and close your eyes for ten minutes. Don't ignore your body.
If your at home and have the option to sleep what I find works for me is lying flat on my back. Then relax, in sequence, your forehead, shoulders, upper arms, lower arms, fingers, chest, stomach, groin, upper legs, lower legs, toes. Take around 1 minute to relax your whole body. If you can't relax something tense it up, then release it.
Once your fully relaxed close your eyes and clear your thoughts. What works for me is trying to focus on a spot in the dark directly in front of me. As your mind wanders pull it back to focusing on the spot in the dark. Sometimes I need to repeatedly relax my forehead and eyes while doing this, just let your eye sockets go limp.
Occasionally when you are pulling yourself back from your thoughts to focusing on that spot in the dark it can be jarring. Just keep trying. Eventually I fall asleep.
I've noticed recently that this is meditation, except without the sleep part.