Hi Max,
I like blackness, particularly at bedtime when I am trying to get to sleep. The problem with solid blacks though, is that they can be very disjointing, and it is for this reason that I prefer to have some flow through my images.
The one simple difference I might have added to this image is a long dark piece of cloth that you could have draped through the image from left to right. In doing this you would have added a nice flow across the scene and aside from adding some flow, it would have created a connection between the two legs.
As for the sleeping comment in your title; if you select the child's face and then adjust the image with curves you can see that they eyes are wide awake!
Hopefully this image will not be too difficult to set up and have another play with!
Cheers for now, Anthony