Actually if you put bub to bed earlier it can sometimes work. It's all about sleep cycles and if bub is coming out of a sleep cycle at 5amish and the light is starting to come in the window they often decide to start their day. Putting them to bed earlier will often mean they are in the middle of a sleep cycle at dawn and will stay asleep a little longer. We used to put our DS to bed at around 7-7.30 and he would get up at around 6.30-7ish (didn't sleep through the night though!) He now goes to bed at around 7.30-8ish and sleeps through to anywhere from 6.15-7am.