Problem with 6600 battery

Hi... I wonder anyone of you has the same problem as me.... Yesterday night before I went to sleep, my 6600's battery life was to its fullest (i.e. having 7 bars FULL). Then, I switched it off and went sleep. Today morning, when I switched it ON, the battery for whatever reason, went down to 4 bars (i.e: 3 bars of battery have gone in one night). How could there be a sudden drop of battery life from 7 bars to 4 bars out of sudden? Mavi forum