Multiple iPhone units stored for forensic analysis have rebooted themselves, causing concern among law enforcement officials that Apple has a new security feature.
I can put the phone in lockdown by press-holding the power button and select the "lockdown" option. Not bad, but an actual reboot daily at night wouldn't be so bad.
It probably requires root, but I'm sure Tasker can do this.
Really, all phones should have the option to reboot into locked mode on certain conditions like being taken out of network or being unattended for a while.