First of all: Why the heck would you want to reboot a router every day. If your router is that unstable, contact the TAC. Or upgrade it. Or do something with the network - whatever.
But since you asked, there are lots of ways:
1. You could run a *Nix script (read: cron job) that shells out and reboots the router.
2. You could use the IOS schedule command and always schedule a reboot the next day.
3. You could use SNMP commands (if you have the appropriate permissions and the right SNMP reload command set in the config) to do it over SNMP.
But seriously: please re-evaluate why you're rebooting your routers. I have routers that have been running for almost two years without being rebooted.
