iPhone use when it's not really being used

I'm going to close your thread and provide you with some links to threads on this topic (I'll be back in a sec with those).

The short answer is: This is a known issue, there are theories about why it happens but no real solutions.

In terms of stop-gap actions, there might be some things you can do about crashed programs, or else you could consider just leaving your phone plugged in overnight when it's not in use.

There are a few other minor possibilities to consider, like the special case where your phone either has no signal or a weak/intermittent signal wherever you leave it while you're sleeping. In that case the phone will drain unless it's plugged in or you put it in airport mode.

EDIT: Links...

http://forums.macrumors.com/showthread.php?t=571212&highlight=iphone+standby+drain
http://forums.macrumors.com/showthread.php?t=564514&highlight=iphone+standby+drain
http://forums.macrumors.com/showthread.php?t=525741&highlight=iphone+standby+drain
http://forums.macrumors.com/showthread.php?t=527000&highlight=iphone+standby+drain