hi hollo
My DS slept on his tummy from birth.
We had the mat in there until he was about 5 months and rolling.

It is so sensitive even under a mattress. Even the fan can be sensed as a breath (you can adjust it though). So you need to be aware of this when it is summer.

But i wasnt concerned as DS had a very strong neck and would turn his head from side to side from 2 weeks old.