It's little hard to find the exact reason for the server reboot after reboots..but as ' Evgeniy_ru' said you can look in to those logs to get some hints..also check for any backup/db dump cron process running during the time o reboots..
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Incase its an issue with certain processes , then neither logs or cron will show anything . For eg: I had a SQL query in my site .. which when called by someone through the site .. will crash the server . Since it happens only at odd times , I had to monitor the server for top, mysql status and apache status one whole day to figure out whats crashing the server.