The next Apple Watch will detect sleep apnea. In 2017, I helped run the first research study that detected sleep apnea using Apple Watch and a deep neural network.

In this post, I’ll explain some of the science behind sleep apnea detection (including techniques like peripheral arterial tone) as well as the potential impact of widespread sleep apnea screening (80% of sleep apnea is undiagnosed).

How on-the-wrist sleep apnea detection works

Obstructive sleep apnea is a condition when you stop breathing during sleep, causing your blood oxygen levels to fall. The more often this happens, the more severe the sleep apnea. This frequency of nighttime apnea events is called the Apnea-Hypopnea Index (AHI).

Traditional sleep apnea tests measure five channels: pulse (heart rate), blood oxygen saturation, snoring, nasal flow, and chest motion. On the wrist, only the first three channels are available.

But two “tricks” help us infer the presence of apnea events without nasal flow or chest motion:

• Peripheral arterial tone. In 2019, the FDA approved the first sleep apnea test, the WatchPAT 1, to use a new technique called peripheral arterial tone. Unlike traditional sleep apnea tests, peripheral arterial tone can be measured entirely with sensors on the wrist. We’ll go deeper on peripheral arterial tone in the next section.
• Cardiovascular signals. Our 2017 study used only heart rate and step count as raw inputs into a deep neural network (an LSTM, at the time). Nowadays, the Apple Watch calculates 20+ metrics of health across multiple organ systems, including several heart health metrics. I just performed an updated analysis that showed 5 of the top 10 predictors of sleep apnea risk are cardiovascular in nature. These heart signals, even if of modest individual accuracy, may help accurately detect sleep apnea when combined.

Peripheral arterial tone & the autonomic nervous system

Peripheral arterial tone (PAT) measures variations in arterial volume, which reflects the activity of the autonomic nervous system (ANS).

The ANS controls involuntary bodily functions, including heart rate. You can think of the ANS as a constant “tug of war” between two halves: the sympathetic nervous system (“fight or flight”) and the parasympathetic nervous system (“rest and digest”). When the sympathetic nervous system dominates, heart rate is higher, heart rate variability is low, and your arteries tensen. When the parasympathetic system dominates, heart rate is lower, HRV is high, and your arteries are less tense.

During episodes of apnea (when breathing stops during sleep), the body’s oxygen levels drop, triggering a surge in sympathetic activity to compensate. This response can cause significant fluctuations in PAT, which allows a wrist-based device to pick up on the apnea event.

In 2019, the FDA cleared the first medical device to use peripheral arterial tone to test for sleep apnea — the WatchPAT ONE. This cleared the way for manufacturers of consumer wearables, like Apple and Samsung, to build algorithms that detect sleep apnea.

Ultimately, the output of these tests is a number—the apnea-hypopnea-index (AHI). The AHI is the number of apnea or hypopnea events per hour. Low severity sleep apnea is 5-14, medium severity sleep apnea is an AHI of 15-29, and high severity sleep apnea is an AHI of 30 or more.

Is peripheral arterial tone accurate for detecting sleep apnea? In a word, yes. A 2013 meta-study showed high correlation (0.89) between PAT and conventional sleep testing. Subsequent studies have validated the technology in patients with COPD and atrial fibrillation.

What’s the impact of Apple Watch sleep apnea screening?

80% of sleep apnea cases are undiagnosed. Undiagnosed sleep apnea affects more than 23.5 million people in the US alone. The complications of untreated sleep apnea—such as car crashes or heart attacks—cause $150B of wasted spend every year.

What to do when your Apple Watch detects sleep apnea

If your Apple Watch alerts you to sleep apnea, or you suspect you’re at risk, you need to talk with a doctor about getting an at-home sleep test.

Book a doctor appointment at Empirical Health — we’re covered by major insurance, available to 200 million adults across the US.