You can manage the urge to sleep with the practice of breath awareness.
1. Sit comfortably erect. Use a chair or a wall to support your spine if helpful.
2. Close your eyes and begin to follow the movements of your breathing. Stay with the breath for a few minutes, until your focus is steady.
3. Without losing your breath awareness, relax your body just as if you were settling it into bed.
4. Continue to follow your breath, maintaining your breath awareness as if it is virtually the only thing of importance in the universe.
5. Relax your mental effort. Maintain a steady hold on your breath, yet relax your body and mind.
6. Now begin to silently recite the mantra so-hum. Let that sound flow with each breath—so on the inhalation and hum on the exhalation. Sense that these sounds are gently blowing away layers of ashes that cover the embers of consciousness in you.
7. Continue for as long as you like. The urge to sleep may come and go, but do not let it dislodge your relaxed breath awareness. Gradually, as you become more rested, your sleepiness will diminish or even disappear. Over a number of sessions you can lengthen the time you sit.