Apple is once again facing user backlash for alarms failing to sound, after an issue with the iOS 9.1 update on iPhones has reportedly led to people sleeping past their scheduled alarm time.

Unlike with some previous iOS alarm issues, alarms failing to sound in iOS 9.1 appear to be just a one-off occurrence. The culprit is believed to be Apple's new overnight update feature, which installed iOS 9.1 overnight on some devices at the user's request, but failed to reactivate alarms after the update was completed.

After users woke up and used their iPhone post-update (tapping through some dialogs informing them of the update), alarms would then work normally. But because the update was performed overnight, some affected users couldn't complete the update process before their alarms were scheduled to go off.

Apple are yet to acknowledge the issue with overnight updates, although they will almost certainly address the problem with future updates. And it wouldn't be the first time Apple had to fix alarms in iOS: iOS 9.0.1 fixed a bug that prevented alarms from going off, while several previous builds (including one from 2010) failed to sound alarms after the Daylight Savings Time switch-over.

For now, if you have an iPhone that's ready to upgrade to iOS 9.1, it's probably best not to perform an overnight update to ensure your alarms will sound.