R.J.
The two times my son did that was:
1) When my school schedule had me away for big chunks of the day... he started waking up in the middle of the night, aparently for "mommy time", because as soon as the new quarter started he went back to sleeping, and when another quarter was like the first there he was awake half the night again. And then back to normal when THAT quarter was over.
2) Whenever he hit a growth spurt. He'd wake up, and unless he was fed, he'd STAY up. As soon as his tummy was full though, he'd go back to sleep. Boy oh boy. Until I figured that one out, I was EXHAUSTED. My son is the "chub up and shoot up" type, too. So his growth spurts were always heralded by him doubling his food intake. <Laughing> It was a pain when we had a substitute Ped. though. I was always having to tell them to look at his weight chart, because they were alternately concerned that he was grossly over or underweight.
It's my experience that non-traumatized children don't have insomnia the way that we think of it. They have a need that needs to be filled.
One of my favorite sayings:
Babies know what they need
Children know what they want
Adults have adgendas
A baby waking up in the middle of the night needs something. It could be food, teething (thank GOD for tylenol), or pure and simple "mommy time". Ahem. Mommy time is the hardest, actually. Once you figure out what it is, you'll be fine.