Unexpected reboots of a Ubuntu Linux

I have a Ubuntu Linux machine acting as a bridge/firewall between the Inteet and my home network. This host works 24x7 unattended. It just sends me a report every day. I've paid attention that in the last several months there were several unexpected reboots of the machine. There were no suspicious logins. I've also checked the /var/log/syslog but could not find anything related (by time) to the reboots.

How can I find the reason for the reboots?