When I described my symptoms prior to BM, my GP was convinced it was to do with vagal stimulation. It's a cycle of severe weakness, then a flush of muscle pain, then a vague sense of doom, then an overwhelming urge to sleep that's so strong I could do it standing up, then profuse sweating when I wake up 10 minutes later. This repeats every 50 to 60 minutes.
The problem is that this should happen only once, and after
the BM. With me, it's always beforehand and often repeats for hours on end. I feel completely back to normal afterwards, but the lead-up (anything from three hours to a full day) I'm fit for nothing.
It can also happen on a lesser scale when I've got a full bladder, though I don't actually fall sleep. It's only when I've done too much activity, though – on a normal day I just flush and sweat profusely.
I've never found a way round it, and have just had to adapt to living with it. Interestingly, my sister, who's never had ME, has the same symptoms with a full bladder. As with me, it started in her late 30s for no apparent reason. So now we both announce a need to visit the loo by going scarlet and dripping with sweat!