I have the same problem with my 98 year old Mom. The only difference is that she does it in the early morning -- around 4 or 5 a.m. And instead of saying "Help" she simply calls my name. And as with your mom, once I'm there she'll tell me she's okay but then will forget and call me again as soon as I'm gone. It drives me nuts. It does seem, however, that if you give her something to eat --a banana, say-- she'll calm down and fall back to sleep.