I used to have restart problems in the heat of summer on one of my machines. Running FAH at 100% would cause spontaneous reboots due to high CPU temperature if it was warm in the house. Doesn’t seem likely this would happen at night like yours, but it might be worth a check.
Do you have a utility to check the temperature of the CPU? I use MBM5 to monitor the problem machine and it can be set to keep a log.
http://www.majorgeeks.com/download.php?det=311
Throttling the CPU to 90% for a month or two in hot weather worked for me until I got a better cooling fan.
My cpu usage for F@H has been running at 95% for over a year now. I'm beginning to think that it's some firefox add-on. My computer went haywire about a week ago and I started a thread on my problem. I removed the many add-ons and my computer stopped shutting down as I slept and the many many other problems I encountered.
For two mornings I woke up and my pc was still running. As I said, I may have to recheck my add-ons and see if one of them is giving me the problem.
I'm gonna scream if I start getting that nasty error message again.