I have samsung galaxy s. my battery drains out in morning completely even I...

...had slept for just 4 hours.? Battery was fully charged. Strange fact is, it works fine during the day. But I guess battery drains out when phone is idle. I have wi-fi sleep policy is set to 'never'. Wifi is off most of the time. Never used google latitude. Can any one tell me please the solution. If it cant be cured should I leave my phone on charge whole night?