My son of 20 months is having the same issues. At 4 months of age he was sleeping through the night and now, for the past few months, waking. He wakes 2-3 evenings a week anytime between midnight and 3 am. Sometimes, many times a night.
Our bedroom is next to his so we say through a closed door, "Honey, it's Daddy (Mommy). Go to sleep. It's night night, very late." It sometimes works and he goes back to sleep. And, it sometimes miserably fails and he continues to cry and whine. After an hour or more of crying, not out of pain but possibly from separation anxiety, we end up going in to change a diaper and make sure he's alright. Daddy started a terrible habit of putting him in front of Baby Einstein DVDs and he's hooked. We've tried to go cold turkey on the DVDs but after many cries out through the night a short DVD and then to bed is faster then listening to him at all odd times of the night or very early morning.
Have you received any advice as of yet? We could really use some.
Sleeping in Denver, Colorado