Both my kids have been marathon night nursers, and I don't think it had or
has ( my two+ year old still nurses 2-3 times nightly) anything to do with
unusual problems like allergies or whatever.  That was they way they were and
they wanted it.

I found that if I tried to takethe breast away too fast they would wake up
screaming every time.  Dr. Sears in his book "Nightime Parenting" explains
sleep cycles, and why it takes them 20-40 min. to get into a deeper sleep
cyclye.  Usually, at this point mom can ever so gently pull the nipple from
baby's mouth.  (It usually hurts a little, because you don't want to go
sticking your whole finger in their mouth to break the suction--that always
wakes them up).  And then mom has to stay laying there, ever so still, for
another couple of minutes until baby is completely settled again.  Then she
can gently roll away.

It may help if mom and baby sleep in their own bed.  Accessory
movements/breathing sounds from dad may rouse baby.

I also found that when I controlled my breathing--slow, deep breaths--my kids
fell into deeper sleeps more easily.

Soft music in the back-round may help too.

All this may very well have something to do with teething.  My kids always
have nursed more frequently and for longer periods during the night in the
couple of weeks preceeding a new tooth. At Age 18 mo.  my daughter went
through a 6-week period of nursing 7-9 times every night--I thought I was
going to lose it. Finally 4 teeth popped through.  How about some homeopathic
teething remedies:  Chamomilla and Calcerea Phosphorica are common options

Finally, if nothing works--how about taking the nipple away after about a
half hour and then rubbing baby's back, rocking, singing to baby to get it
used to sleeping without the breast it its mouth.  I know how frustrating
this can be.  I've never been able to just sleep through night-time feeds,
and when you are exhausted this can do you in.

Highly recommend Dr. Sears book: Nightime Parenting

Kim Connor PA-C, CLE  (mom who hasn't slept more than 5 hrs straight in over
4 yrs.--ahh the joys of motherhood)