This is one of the more genuinely useful things Apple has shipped in health, and also one of the most misread, because people expect a nightly number and it is not that.
What it actually measures
Breathing disturbances: small interruptions in normal breathing detected through the watch’s accelerometer picking up the movements associated with them. Every night contributes a data point.
Rather than reporting nightly, the watch accumulates those over a 30-day window and looks for a consistent pattern. If one emerges that matches moderate to severe sleep apnea, you get a notification. If it does not, you get nothing, which is why most people never hear from it.
The 30-day window is the important design decision. Everyone has some disturbed breathing on some nights, particularly after alcohol or with a cold. Requiring a sustained pattern is what stops the feature generating constant false alarms.
Which watches have it
Apple Watch Series 9 or later, Apple Watch Ultra 2, and Apple Watch SE 3, on current watchOS. Earlier hardware is excluded and no update changes that.
You also need to actually sleep in it, with sleep tracking on, often enough to accumulate the 30 days. Which model gets you what across all the sleep features is in the best Apple Watch for sleep tracking.
If you get a notification
Take it seriously and do not panic. Sleep apnea is common, very treatable, and frequently undiagnosed for years, which is exactly the gap this feature exists to close.
The watch produces a report you can share with a doctor. Bring it, along with anything you know about snoring, witnessed pauses in breathing, morning headaches or daytime sleepiness. The next step is usually a sleep study, which is increasingly done at home.
If you never get one
That is the expected outcome for most people and it is good news, with one caveat worth holding onto: absence of a notification is not a negative test result. Mild sleep apnea can pass unflagged, and the feature is one limited instrument.
If you snore heavily, wake gasping, or are sleepy during the day despite adequate time in bed, that combination is worth a medical conversation whatever your watch has or has not said. It is also worth ruling out before you spend months trying to fix a poor recovery score with sleep hygiene.
How it relates to the other overnight metrics
Respiratory rate, which the watch also records, is a different thing: an average breaths-per-minute figure that can look entirely normal across a night full of apnea events. That is why apnea needed its own feature rather than a threshold on an existing metric. More on that in respiratory rate while sleeping.
The other 30-day screening feature is hypertension notifications, which works on a similar principle and is misread in a similar way.