J.L.
i don't remember exactly when, but i think around 7 months, my son started waking up after only sleeping a few hours. it was driving us crazy, because he was sleeping 7-8 hours consistently. after talking to the pediatrician, we decided that if our son didn't sleep at least 5 hours, we weren't going to feed him. we'd comfort him while he lay in his crib, but we didn't feed him, which we had been doing.
only 3 nights of this, and he stopped waking up after 2-3 hours. he started sleeping through the night again.