On a typical night, I go to bed wearing a tracking watch on one wrist and another type of tracker on the other wrist. Then I compare their sleep findings while I drink my morning coffee.

I direct the Center for Modern Digital Health at Carnegie Mellon University in Pittsburgh, where my team and I study how to use wearables to improve health and performance. My lab’s research has relied on consumer devices for years. I wear them for both science and pleasure, and I also wonder how much of what they tell me is true.

Apple’s newest watch, announced Sept. 9, 2026, greets its wearers each morning with a single “readiness” number, built partly from how it thinks they slept. Apple says the redesigned sensors deliver the most accurate heart-rate tracking of any wearable, but independent labs have not yet confirmed that.

Given the growing popularity of sleep trackers, I think it’s worth asking whether a sensor on your wrist or finger can really tell you how your brain was sleeping last night.

Here’s what recent studies show when consumer wearables are tested against the gold standard, polysomnography.

Does it know when I was asleep?

Polysomnography, which combines EEG, or electroencephalogram, with other sensors, records brain waves, eye movements and muscle tone while a person sleeps.

On the simplest question – asleep or awake – today’s devices come close. A 2025 meta-analysis of 24 studies and nearly 800 participants found that wearables typically overestimate total sleep time by about 17 minutes compared with polysomnography. In an eight-hour night, that’s an error of under 4%, roughly two taps of the........

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