I just thought I would post the following pictures. As you can see, our Princess Sansa (a.k.a. Sansa the cat) does not really understand the fundamentals about mattresses.
If there is a 'wrong side' to sleep on a mattress then she has found it.
But then she may have a different opinion. She may think that the way us humans (hoomans in cat speak) use mattresses is just not enough of a challenge—and cats like a challenge.
Look at the way she has her 'little' toe hooked into the outside binding of the mattress.