How can I prevent my Desire HDs battery from draining overnight? I've recently got the new HTC Desire HD, which has a poor battery to start with (1230 mAh), but in the last two weeks I've had several nights waking up with a dead battery.

The first few nights, I suspect that wifi was still running and by default the radio didn't go to sleep when it's idle (changed that). I reduced syncing frequencies for most services to once a day, so that can hardly be the problem. When looking at the battery usage reports, it's mostly the huge screen draining the battery, but that's turned off at night. Last night I turned off any data connection and went to bed with 90% battery and woke up with a dead one. 

So what on earth is crashing my battery so quickly? Does anyone have any suggestions other than the no-brainers mentioned here.
I'm suggesting an app called PowerTutor (free on the Market). When you turn on the profiler, the app monitors the power usage of each applications and android subsystem, and give a detailed report of what application has been consuming the most battery. 

The idea is, you turn the profiler on, then use the phone as usual. Be aware though, the power profiler itself is very power-hungry and it does not save the profiling result if the profiler is restarted, that means if your phone's battery runs out before you can look at the report, you'll just be wasting battery (IOW, you might need to wake up in the middle of the night to look at the report before your phone runs out of battery; or you might need to put the phone on charger, which might affect the result of the profiling since some applications might behave differently when charged and when on battery).