Is this NORMAL for my dog?

Okay, so I have had my female papillon for three years and have two other dogs and I was wondering if this is normal; sometimes when my dog goes to sleep she kind of whimpers and starts moving, as if she has a nightmare. Is this normal? It's been going on for a while, I think it's normal, but I just want to make sure =) When she wakes up, she is normal and healthy and he usual bubbly self. Thanks