She drinks properly with sucks and swallowing while still kind of asleep. She always has. I unlatch when she starts light/butterfly sucking - if I unlatch when drinking properly with eyes shut there are major tears! Also these are needed feeds - I battle to get her to put on weight and bf advisors have said not to cut short any feed where she's drinking.
Mine did this a bit around the year mark and still does the odd time by mistake. I tried the unlatching and a firm "No!" advice but that just made her giggle and try again.
So I'm just extra vigilant when she finishes the actual drinking and/or gets sleepy.
She has a soother so once drinking stops I'll pop a finger in to unlatch and replace with soother. If soother isn't to hand I'll tend to unlatch and then hold her in close to my chest to stop her rooting. Otherwise I just leave her comfort sucking and have finger ready as soon as I feel even the slightest bit of a bite. Believe me, you'll become a pro at pre-empting it.
I haven't had a proper bite in a good while (touch wood) but have tended to just gasp and remove her gently and say something like "oops, careful DD, not to worry" and then carry on. She seems more concerned when I gasp as opposed to say "no", as I think it gives her a little fright too so she does seem to be more careful then.
Oh no, poor you - that sounds really painful. We had this briefly when my DD was about 8 or 9 months corrected.
During the day, I'd say NO, unlatch her, put her down in her cot/playpen and leave her there on her own for a few minutes. There were tears - I'm normally a softie, but I felt that it was important to make the point strongly.
I was hesitant to do the same at night the first time it happened (as she was an incredibly bad sleeper and I couldn't face doing anything that would wake her up), but then ended up doing it the next night, as I felt it was important to be consistent. It seemed to work within a couple of days.
Day or night, I just stopped the feed and never relatched, but I do take your point about weight gain. I sympathise, as we've also had up-and-down weight gain. It's a tricky one. I've read about a technique where, when they bite, you kind of squish the baby into your breast so as to block off their nose, which forces them to open their mouth. It might be a bit gentler for nighttime use!