This is the kind of software I'm now seeking. My finds:

Keep It Up: site is down. Google apparently knows nothing about it. Wayback Machine = also nothing. (Remember the Color Classic web server?)

Cron Manager: buyware. Across irrelevant things, I found a MacWEEK article on one paid subscription based library and other which requires you to enter your postal address. I think the text won't be useful.

By preference it should reboot on anywhere between midnight and 6am.