When we used to have the Bronx class on Monday nights in the Temple of Enlightenment, Rev. Ji Ru would teach and lead us in meditation for the last 30-45 minutes. I distinctly remember someone asking that very same question about feeling sleepy/falling asleep during mediation, and the Abbot's response was that sleepiness during meditation was one of the "evils".
The way I approached it was by treating sleepiness as I would treat my mind wandering during meditation, that is by paying attention to the method, and keeping alert and "watchful". I never let myself just relax and "chill", but tried to practice with the same mindfulness as when doing standing or dynamic meditation. I know it's more difficult when doing seated meditation, and if you are tired and sleepy to begin with, even tougher. Maybe try watching your mind and body with your "mind's eye" more closely, and if you catch yourself relaxing and dozing off, go back to the method. Hope this makes some sense.