Daniel did EXACTLY the same thing. We just persisted with letting him know that we are running the place - not him, and that he is in bed to sleep.

It seems to have worked.

My CHN said it is because at about 5 to 6 mths they start to remember that they like you and that you aren't around - separation anxiety.

I just go down to his room - say soothing things, put his dummy back in and say "goodnight" every time. He soon went back to sleeping through again. It took a couple of months though.