I use a Nagios
a network monitor that pings the TiVo's IP address then logs and sends me a e-mail alert if several of pings are not returned. Nagios was a little difficult to setup ~10 years ago probably much easier now. I'm sure others will chime in with there favorites.
Check the e-Sata cable between the DVR and Expander for a tight connection replace if not a tight fit. A loose cable can cause the DVR to reboot. Do you have a cat that likes warm places?
Interesting that it only reboots at night. It may be a good idea to put both the DVR and Expander on the same power strip and definitely get a good U
upply. I had to get one here because at 7:45 almost every morning there is a mains power switch that caused A PC and one of my TiVo's to reboot the drop did not cause problems on other electronic devices including another TiVo plugged into the same outlet alto that may of been an early symptom of a failing PS that I had to repair a year later.