I'm pretty sure its more economical to keep the heating on all the time but on a lower setting then to have it come on and off anyway, at least thats what I've been told.
We gave this a try and I have to say our bill went up massivly, it seemed to help my daughter sleep though so last winter we did leave it on all the time. It was worth the money just for a few extra hours sleep lol
Cant really afford it this winter, and plus she doesn't sleep great whether shes warm or cold so not sure it would help now anyway