Users have been complaining they were missing their alarms.

"This has probably been the third or fourth day in a row that my alarm clock has not gone off," one TikTok user Reid Manning said.

It is unclear how many people have been affected.

It has also not been confirmed if the bug is restricted to particular iPhone models or a specific version of its iOS software.

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Tik Tok user Alyssa Sarracco, missed classes after taking a nap even though she set "five alarms". "This is not me," she said in a video that has amassed 3.7 million views. "I am not the problem here."

Apple says it aware of an issue causing some alarms on iPhone to not play a sound as expected, and is working to fix it.

Some people are blaming the phone's "attention aware" feature which lowers the sound of alerts if its user is looking at the device. By switching the feature off, some users say they have managed to bring back their alarm sounds.

To turn off the feature, go to iPhone settings, tap 'Face ID & Passcode' and scroll down to find 'Attention Aware features'.

They are available on iPhone X or newer devices, and iPad Pro 11-inch or iPad Pro 12.9-inch, Apple says..

Users are being encouraged to double-check their alarms before going to bed - ensuring their settings are correct and the volume is turned up.