take it you don't have anything running on it over night

if the screen is off and there's nothing running the processor should be @ 0Mhz

if there is something running or its connected to gprs/wifi it will be draining the power once its charged
if you plug it in @ 11pm and its full by 1am, even tho its still plugged in, its no longer charging so any connections, running apps or even missed calls/sms will be eating power