2 months: One waking again?

My two-month-old twins were sleeping from 11 until 5 for almost two weeks when one of them, the boy, started waking again at 2 or 3 every morning. I'll nurse him, (and let her sleep) but he still won't go back to sleep for hours. Has this happened to anyone else? Why? And what did you do about it?

 

That happened to us although mine were a few weeks older than yours. I decided that since she was capable of sleeping that long before, I wasn't going to respond straight away next time... So I gave her about 5mins to resettle before going in, a few minutes longer the next time and within days, she was back to sleeping through again. Most likely a sleep cycle issue. In saying that she didn't really cry when she woke more like fussed so it was easier for me to do this.

We still occasionally have this, especially when they haven't been napping very well. We never go in, ever (they don't cry, just play/talk) and we have taken away the night light too. It is always the same time so I know that it's a sleep cycle.