Anyone who has meditated will have met this one at some time or another: the irresistible urge to fall asleep! If you are sitting in an upright posture, you won’t actually drop off, but rather keep starting to flop and then waking yourself up with a start as you do so. I find it a painful, uncomfortable state to be in: not sleeping and not meditating either, but unhappily careering from one to the other, and feeling confused and trapped in the cycle.
That experience is sloth, which (strictly speaking) can be distinguished from torpor. Torpor is not exactly falling asleep, but hovering in a sort of blank, half-resolved state just short of it. I haven’t really experienced torpor much myself, and sloth seems to be very much the product of specific circumstances. So one of the best things one can do about sloth, in my experience, is just to avoid those circumstances. It’s just a list of meditation no-no’s really:
- Don’t try to meditate straight after a meal
- Don’t try to meditate after consuming alcohol, even a small amount
- Don’t try to meditate after a long walk or other soporific exercise
- Don’t try to meditate lying down
Of course, your experience may be different. You may be able to break all of these rules. But my experience of thinking “I don’t need to worry about that: it was only a small glass of wine/ I’m not sleepy really/ I don’t need to be so rigid about this” and attempting to meditate under any of these circumstances, is that it really doesn’t work.
Then there’s the afternoon sag. Perhaps it’s later on in the afternoon, and you’re on retreat, so you sit down to meditate with everyone else because it’s on the programme – but then the irresistible tentacles of sleepiness begin to creep around you and gradually haul you towards them. That octopus of oblivion is just about to engulf you when… Oh yes, I was supposed to be meditating! But the afternoon octopus only goes and hides behind a weak intention for a short while. He’ll be back shortly. 
There are only two ways I know to avoid the afternoon octopus. One is to drink the right amount of caffeinous drink beforehand, so that you’re awake but not over-stimulated. The other, probably more wholesome method, is to have a preparatory afternoon nap.
There are lots of other ways you’re supposed to be able to deal with sloth and torpor. Imagine lots of cold water splashing on your face. Raise the awareness higher in your body. Even visualise your body as full of light. None of these really work for me. In some cases, a degree of sleepiness may just be a way that some other kind of resistance is expressing itself, and if you just work through it, suppressing (but not repressing) the sleepiness, you might end up having an especially rewarding meditation because you’ve found a way of integrating that resistance. But in my experience, that’s exceptional. Most sleepiness is just about the immediate physical situation or one’s immediate bodily state. The usual solution if all else fails is very simple: get up, go off and have a nap!