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iPhones stored for forensic analysis unexpectedly reboot, causing problems for officials
(appleinsider.com)
This is a most excellent place for technology news and articles.
There is a shortcut action to shut down the phone which you could trigger with an automation, I suppose.
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.
You can, see my other comment: https://feddit.org/comment/3001525
As the linked screenshot shows, you have the option to choose between shutting down and rebooting. There is no need to explain the difference to me, I demonstrated that the thing you want to do is possible.