Lethargy: Short Versus Long Sessions

I have roused myself to meditate, but I am experiencing lethargy and drowsiness throughout the whole session, drowsiness which doesn't cause me to sleep but which I perceive helplessly. My question is should I: a) continue to merely watch my hazy thought process as long as I am not sleeping b) reduce the length of my sessions so that the *time* spent in meditation is devoid of drowsiness. I've done a session of 30 min, and am wondering whether to reduce it to 15 or 20 min. Thank you.