There are a couple of reasons why Mirra normally sleeps inside the mirror:
1) She is not used to sleeping in human form/world
2) She never slept on a bed… so she moves and falls off of them like a baby without supervision.
Still, there are times on which she feels too tired and just falls asleep on whatever comfy place she is in… needless to say she wakes up with her body hurting XD
athorment 