I don't use a sleep mat as I have a couch in the playroom which has in the past accommodated 2 children with no problems. If the child is able to follow instruction and understands that it is time to sleep/rest and that they must stay where they are laying till I come and get them then that is when I start them on the couch.
Here, to be allowed to sleep on the couch is considered a big deal, so the kids have always been good so they don't loose their treasured couch spot.