Sleep tracking apps promise better rest through data. But what happens when the pursuit of a perfect score keeps you up at night? For a growing number of people, that Apple Watch, Oura Ring, or whatever device meant to improve your sleep quality may be doing just the opposite. Here’s what to know about how sleep scores really work, and what you can do to make the most of your sleep tracker.

The benefits (and accuracy) of sleep tracking

Sleep is foundational to almost every dimension of health, including “improved mood, heart health, and cognitive function," according to Dr. Rebecca Robbins, Assistant Professor in the Division of Sleep Medicine at Harvard Medical School and an Associate Sleep Scientist at Brigham and Women's Hospital. On the flip side, chronic sleep deprivation is linked to increased risk of cardiovascular disease, metabolic dysfunction, impaired immune response, and mood disorders.

For decades, however, people had a surprisingly poor grasp on how much they were actually sleeping. Self-reporting is notoriously unreliable; we tend to round up, conflate time in bed with time asleep, and forget our nighttime wake-ups entirely. “Consumer sleep tracking has begun to close that gap with significantly increased precision and accuracy in recent years, providing more insights from home," Robbins says. The Oura Ring and Whoop band dominate the scene, but watches like Fitbit or Apple Watch work too.

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According to Robbins, wearables (like wrist- or ring-based devices) are equipped with multiple increasingly precise sensors: a temperature sensor, an accelerometer for movement, light sensors, and photoplethysmography (PPG) technology. (PPG uses pulses of light to detect blood movement beneath the skin, allowing the device to calculate heart rate and approximate blood oxygen saturation).

Your device tracks how long you seemed to be asleep, and makes guesses as to how much of that time was spent in light sleep, deep sleep, and REM sleep. Then, it distills it into a single composite score. It's a cool number to have, but it’s important to remember that this number is an approximation, and each company has its own grading system of sorts.

There's also the question of what "quality sleep" even means. The clinical definition centers on adequate duration (most adults need between seven and nine hours), sufficient continuity, appropriate sleep stage distribution, and, critically, how you actually feel upon waking and throughout the day. That last part is, ultimately, a subjective experience—in other words, something no wearable can measure. The person who wakes from an uninterrupted eight hours feeling groggy and unrested is not having quality sleep, regardless of what their app says. And conversely, someone who wakes feeling sharp and energized after a night their tracker graded a 65 should probably trust their body over the algorithm.

This is why it's impossible for a sleep score to be truly "accurate." Yes, all the data that going into your score (like your heart rate) might be accurate, but it's important to understand that the score itself is a made up number. Different companies have different definitions of "good" sleep, which vary from device to device. For instance, Oura and Apple both give scores ranging from 0-100, but where Oura labels a score of 70-84 as "Good," Apple has a range of 61-80 labeled as "OK." These scores aren't what you'd call clinical precision.

Still, for most people, clinical precision isn't the point. If the point is to get some behavioral feedback, then your smartwatch or sleep tracker is a great tool.

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Wearables can be useful to facilitate changes in habit

Sleep tracking, at its best, functions less like a medical test that you pass or fail, and more as a way to see patterns over time. Maybe you’ll start to notice the creeping effect of a late-night glass of wine on your deep sleep percentage, or the way your resting heart rate climbs after three nights of cutting things short, or perhaps the concrete difference a consistent bedtime makes over weeks.

"People are highly motivated by their scores," Robbins says. "Wearables can foster intrinsic motivation for behavior change by providing daily feedback." In other words, your score (good or bad) can prompt you to reflect on your actions: What did I do yesterday? What can I do differently tonight?

What matters most here is that you don’t need to “optimize” every little stage of your sleep architecture to benefit from this kind of tracking. "The most powerful use of this data is when people can monitor their progress over time,” Robbins says. Rather than obsessing over your score night-to-night, you should focus on whether you're trending in the right direction.

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