My daughter does this too. She is 2. I notice with her she that now does it when she is upset. But I was glad to see this post, because I had thought it was so unusual.
I told my peditrician when she first starting doing and he said not to make a big deal of it at all, or draw to much attention to it.
However, he also said if she continues to do it could be a problem and that some children do this. If it is only at night perhaps it is teething, or maybe she feel insecure at night? or trying to get attention? or likes to have things in her mouth? Does she suck her thumb or use a pacifer?
My daughter was doing this habitually when she first discovered how to do it, but now she does it only very infreguently - if she is really upset.
I will be curious to see what responses you get, as it kind of completely baffles me. I usually try to distract her. We have this game where she likes me to kiss one foot that the other. She thinks me kissing her feet is very funny, and she usually forgets about it.
I'd talk to your peditrician. Mine's advice of not making a big deal of it worked for me. He said try to distract, figure out why, but not to tell her No No as it will only draw attention to it even more, and you dont want it to become a ingrained habit.
I think that the discover they can do it, so thats why they do. My daughter started at about 15 months too. She use to do it alot, I think just because it was something she had figured out how to do. And then she kind of forgot about, and now for some reason I only see her do it when she is upset. This new habit actually concerns more more. Before I think she was just experimenting. This is not something I want her to do when she is upset.