I know some of the other suggestions imply that these kids may be waking and climbing out of bed, which could be true, but others just sleep like this. My oldest, who is now 9, still falls out of bed all the time - and I know this because I have a t.v. monitor in their room - as well as witnessing her in hotel rooms when we travel (where I have usually put the mattress down on the floor for her). She is a wild sleeper, and no matter what, falls out all the time. She'll even fall out from the head of the bed, if it's designed without a headboard. So, having said that, you may not be losing much to invest in a twin bed with side rails now, because it is possible you'll be using them for a long time, if your son is also a wild sleeper. My daughter agreed to have rails on the bed until she was 7 years old, at which point she switched to a bed that is practically on the floor itself, so that when she does fall out, she doesn't get hurt. I myself was a wild sleeper as a child - so maybe it is genetic -although my youngest child sleeps like a rock. Good luck.