Putting the phone on Airplane mode will drastically improve battery life, that is if you don't need service overnight, but your problem looks more like a previous poster said, software glitch, or faulty battery.
Did you leave any apps that make use of the "standby mode" active in your multi-task menu? Maybe one of those has a glitch and is running your battery dry quicker than it should. Close them all, get the battery to 100% and then see what happens.
I just leave my phone on charge all night so I have 100% in the morning .
I've been fiddling a bit with doing a hard boot of my iPhone 4 (hold both sleep and home buttons down until I see the Apple logo) and then testing frequently used apps to see which one is eating my battery (even though I quit the App by pressing the home button a single time). I've got it narrowed down to two possible suspects: The 'Facebook' app and 'Angry Birds'.
Beyond Mail and Safari, those are the two apps I use most often. Between the two, I'm leaning toward 'Angry Birds' as being the possible culprit. More testing is needed though.