How Apple Watch app is able to 'know when you're sick' days before people actually experience symptoms

Here is how the Apple Watch Vitals app is able to tell 'when you're sick' days before you actually start experiencing symptoms.

Earlier this week, Apple Watch users found themselves pleasantly surprised after discovering one of the features from the newest watchOS update meant the Vitals app was now able to tell if your body was beginning to exhibit signs of possible illness.

Now, health features on smartwatches aren't anything new, with users of Apple Watches, Fitbits and Garmins already able to keep track of their pulse and daily step count. Which is fun for the first five minutes after unboxing the watch.

The Vitals app features have caught the attention of Apple users (Apple/Alex Segre/UCG/Universal Images Group via Getty Images)

However, the addition of Vitals - which is available on Apple Watch models Series Eight and later - has captured everybody's attention just a month after the update became available.

And it seems for good reason as well.

Taking to Reddit, one person revealed the app had been able to work out they were unwell three days before they began to experience symptoms.

The user wasn't the only person whose benefitted from the update, with another commenter revealing they'd received a notification about health outliers days before feeling ill.

So how does this work?

How your Apple Watch can detect signs of sickness

Apple hasn't commented specifically on the addition of Vitals, however, it would appear the app uses data obtained from your sleep in order to obtain the information.

Guidance shared on Apple's user guide stated the feature is available to anyone who lets their watch track their sleep data, with the information then used to gauge your overnight vitals such as heart rate, blood oxygen* respiratory rate, wrist temperature and sleep duration.

In the Reddit user's case, their watch had recorded a high body temperature.

Users can then opt to turn on notifications if their watch takes measurements outside of their normal range.

Unfortunately, the app can't prevent you from getting ill - we're still a couple decades of medical science away from that.

The feature isn't intended for medical purposes, it's important to note (Apple/Alex Segre/UCG/Universal Images Group via Getty Images)

However, it can be a handy heads up on what's to come.

Redditors seemed to be in agreement about the app's usefulness as well, with one person writing on the thread: "this is my favourite watchOS 11 feature," while a second added: "Kinda insane how useful this feature can be."

However it's worth noting that your watch shouldn't be used for medical purposes and isn't a substitute for seeing a doctor if you have serious health concerns.

*Blood oxygen monitoring is not available on models sold in the US on or after 18 January 2024.