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

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

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

However, the explanation is actually much simpler. Referring to the feature as an "inactivity reboot," AppleInsider says that this reboot timer isn't based on network connectivity or the phone's charge. Rather, the reboot simply occurs after a specific length of time -- around 96 hours.

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.