Baby rolling at night!

My almost 5 month old just starting rolling from back to tummy at night this last weekend. He rolls and just keeps sleeping even when he is facedown on the mattress. We have an Angelcare alarm and a breathable mattress, but I can't help worry and roll him back over. The main thing for me is that I've never seen him roll tummy to back, so I don't like that it seems like he's stuck, although he seems comfortable, or at least too tired to care.

Anyone else having this situation? Or if not, would you just let your baby sleep that way?