The Apple Watch’s new Vitals app will only work properly if you wear it while sleeping, Apple says

At its Worldwide Developers Conference (WWDC) in June, Apple lifted the lid on a new Apple Watch app called Vitals that’s arriving in watchOS 11. Designed to help you track your health metrics, and act on them, it was one of the more noteworthy new additions to the Apple Watch announced at the show.

One thing that Apple didn’t mention, though, was that you’ll only get the full benefit of the Vitals app if you wear your Apple Watch while you sleep, which could mean some big changes to your routine if you usually leave your Watch charging overnight.

In an interview with CNET, Dr Sumbul Desai, Apple’s Vice President of Health, said that wearing the Watch to bed is important because it lets the Vitals app understand stats like your heart rate, body temperature, and more without interference from exercise and other daytime activities. The Vitals app needs a complete picture of your health – including what’s happening at night – in order to determine if your measurements are normal, or deviating from your base levels.

“So much of your health is invisible,“ Desai said. “This daily health status is almost like a little bit of a snapshot of your health overnight.” Instead of giving you a health score, Vitals works out how your current metrics compare to your regular levels, and then, if two or more of those metrics are out of whack, tells you what you can do to get things back to normal.