Its hard for the first few night, constantly putting them back in bed and telling them they have to wait till the sun comes up.
But the grow clock comes with a book and a demo mode so that you can explain it all to them before bed.
My DS still wakes at about 5, sometimes earier (we can hear him playing with his cars in bed lol) but he knows he is not allowed "up" until the sun comes up. So I dont really think it changes their sleeping habits per se, it just trains them to stay in bed until they see the sun. (but hey, that works for me )
Every morning without fail, we greet the day with the yelling of "the sun's come up!" Its so cute.