Wearable Health Technology: What Is the Data on Your Wrist Telling You?

The small devices we wear on our wrists, our fingers, even our arms now hand us a "health report" every morning: a sleep score, a recovery score, a stress level, a fitness estimate… But what do these numbers actually tell us?

What's measured, and what's estimated?

It's important to draw one distinction up front: Some values are measured directly, while others are estimated by algorithms from those measurements.

Metric	How it's obtained	Overall reliability
Step count	Motion sensor	Good for daily trends
Resting heart rate	Optical sensor	Generally good
Heart rate variability (HRV)	Optical sensor, mostly overnight	Useful for tracking your own trend
Sleep duration	Motion + heart rate	Reasonable
Sleep stages	Algorithmic estimate	Limited; may differ from lab measurements
VO₂max	Estimated from heart rate and movement	A rough estimate; useful for trends

The most valuable use: Your own trend

The greatest strength of wearables is that they show not a single day but your trend over time. If your resting heart rate has been dropping for weeks, or your step count is climbing steadily, that may be a meaningful signal.

A single night's "poor sleep score," on the other hand, is no reason to worry. Some experts even warn that fixating on sleep scores can heighten anxiety and disrupt sleep.

Why does VO₂max matter?

VO₂max is the maximum amount of oxygen your body can use during exercise, and it's a measure of cardiorespiratory fitness. Higher fitness levels have been linked to longer life in many observational studies. While your watch's estimate isn't as precise as a lab test, tracking how it changes over time can be motivating.

How to be a mindful user

1. Look at the trend, not a single day.
2. Turn data into behavior: What is your sleep data telling you? To go to bed earlier?
3. Listen to your body: If you feel good, don't let a low score run your day.
4. Don't leave medical decisions to a device: Always have alerts such as an irregular heart rhythm notification evaluated by a doctor.
5. Don't forget privacy: Check where your health data is stored and who it's shared with.

Data is a compass; walking the path is still up to us.

In The Discovery List series, we explore new technologies not because they're popular, but for what they truly offer.