It was when mine started doing this that they were banished to their own cot I'm afraid to say, DD is in a travel cot by the side of my bed. You don't have to put a mattress down though, a folded up quilt or blanket on the floor around the bed is soft enough, and you can roll up a blanket or towel and wedge it in the gap between your bed and the bedside table to stop DS sliding down there. It worked with DS2 when we moved him into the bottom bunk, I was worried about him being in a big bed as he was only 18 mths, but it worked!