Ubuntu restarts nightly at 12:00 EST. Why?

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http://askubuntu.com – Title says it all really. But every night, unless I'm actively using it, my computer restarts, and every time, at midnight. I was wondering if their is anything I can do to stop it, or to find out what's actually causing it, or any help in general. I'm using 12.04 LTS. (HowTos)