Ah! That is a common event. I have this, too.

For me, for a while, while working on inner heat concentration, I was generating inner cold for about two months! Along with inner cold, I would get extremely fatigued. It got so that within less than a minute, even in group mediation where there is some bolstering effect of a group even, I would get very cold, start shaking and want to go to bed.

I did go to sleep on most occasions, then stopped meditating for a week.

Then, I thought, "This is nuts! You have to get through this", so I sat and stayed awake, tested a statement in my head (I was going to go through the knowledges of suffering by stating, "I am misery, I am averse...", but as soon as I mentally thought, "I am afraid" the cold and fatigue stopped and heat came up!)

I am not saying that the cause of your sleepiness has the same underlyer or resolution as mine, I am just sharing that I understand the challenge and the sense of having had an [apparently] worthless effect and a drowsy foe!

I think the general encouragement is to try again. In my case, taking a week off out of frustration (and some disgust) was useful. When I sat again, I was alert and determined, not beating myself up every day. It was useful to take a break and let intention re-build.

Good luck.

edit: brackets

edit 2: more from the same monk as above and on drowsiness now. Also, why is it a problem that productive meditation occurs after a nap? That sounds reasonable; a person has some needs for rest.

HI N A -

One thing I paid attention to this morning was how my visual field and drowsiness were related. For example, as my visual focus blurred (and the object of my gaze was a point in the broad landscape to take advantage of the full moon light) drowsiness came on very quickly.

At that point (the point of blurred vision-and-drowsiness), I put attention in the sound and body-sensation (not quite tactility) faculties. Thus the ticks form the heating pipes became significant in my senses (where were those senses before? This relates to mental bandwidth and perception...) and alertness was restored. I returned my gaze to the object and everytime drowsiness came on (with blurred vision) I moved attention to another sense-faculty to restore alertness. I repeated this often for a period.

One thing I notice happens just as concentration (jhana) starts to trigger and build is that sinuses open a touch. There's literally a little click. This may be a temporary event, but I have noticed it over several months (Bruno mentioned something about sinuses months ago and I related to his comment, but vaguely, so I have been looking at it for a while).

Another thing that benefits my practice, though it requires a commitment at the expense generally of other activities (aka: time commitment) is to sit for one hour (and this time may reflect my current aptitude/lack of consistent practice over recent years not any requisite inherent time commitment as far as I can tell). Then move about for a few minutes. Then sit again (maybe 20 minutes or as long as one can/wants). I find the second sit is much more dialed in to the concentration developed in the first sit. Thus, the second sit can be entirely concentration whereas the first was mostly attention-herding.

Good luck.

Edit: adverb replacement