How to determine what causes random shutdown?

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http://forums.fedoraforum.org – I am currently running Fedora 16 with the latest updates installed. I sometimes leave my laptop on during the night, however it often randomly shuts down during the night and I cannot determine why it is doing it or what causes it. What trouble shooting / log grepping can I do to determine why it randomly shuts down and also determine what the root cause is ? I have not been able to catch it in the act, as I say it usually happens during the night and I see it is dead the next morning :confused:. (HowTos)