At first I used to over-sleep (more than 6 hours!!!), so my grades were rarely good. However a teacher of mine advised me to go to sleep early and wake up early, as there are less disturbances in the early hours. And it should be noted that sleeping early does not necessarily mean sleeping less, i.e only 3 hrs or 4 hrs. But we can sleep our full 6 hrs, we only have to set a good sleeping time and a waking time!!!