Inactivity reboot" effectively puts iPhones in a more secure state by locking the user's encryption keys in the iPhone's secure enclave chip.

With iOS 18, Apple introduced a feature that causes the iPhone to reboot every three days, security researchers have confirmed (via TechCrunch). In

A new security feature on the iPhone with iOS 18 reboots the phone after 72 hours of inactivity to protect your data.

Introduced with iOS 18, the new feature automatically reboots an iPhone if it's been asleep and in lock mode for an extended period of time. After a reboot, an iPhone becomes more difficult to crack since either the passcode or biometric verification is needed to unlock it.

Apple covertly inserted a new security feature in iOS 18.1 that automatically prompts iPhones to reboot after 72 hours of inactivity. Read more here.