Apple says it is working quickly to fix an issue that caused some iPhone alarms not to play a sound, giving their slumbering users an unexpected lie-in.

For many people, their phone is an indispensable alarm clock and some over-sleepers turned to social media to vent.

One TikTokker complained that she had set "like five alarms" and they didn't go off.

Apple has confirmed it is aware of the issue - but is yet to spell out what it believes is causing it, or what users can do to avoid a late start.

It is also unclear how many people are affected or whether the problem is restricted to particular models of iPhone.

Concerns were initially raised by the early-risers on NBC's Today Show, which broke the news.

In the absence of an official fix, those losing sleep over the problem can try a few simple solutions.

One is to guard against human error - so double check the phone's alarm settings and ensure the volume is turned up.

Others though have pointed the finger at Apple designers, saying a quirk of the iPhones' "attention aware features" could be to blame.

When active, these enable an iPhone to check whether a person is paying attention to their device and - if they are - to take action automatically, such as lowering the volume sound of alerts, including alarms.

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

Some TikTokkers suggested that if a slumbering user's face were turned towards the screen of a bedside iPhone then, depending on the phone's settings, the features could be activated and alarms silenced.

Apple says it hopes to remedy the issue rapidly.

But, until then, its time zone-spanning customer base may need to dust off a bit of retro tech, and swap TikTok for the more old-fashioned - but dependable - tick-tock of an alarm clock.