What wearables can and can't tell you

A wearable makes building a baseline almost automatic: it tracks while you sleep, train and go about your day. That’s useful. But the numbers on the screen aren’t all equally reliable, and it helps to know which ones to lean on.

What they actually measure

Most wearables measure a few raw signals directly, such as movement, heart rate and sometimes skin temperature. Everything else is estimated from those signals using the company’s own formulas. Steps and heart rate are measured. Sleep stages, “readiness” or “recovery” scores, and calorie burn are calculations built on top.

The more steps between the sensor and the number, the more room there is for error.

Trends beat single readings

Any single night or day can be off: the device was loose, you slept in an odd position, the battery died at 3am. Over weeks, those errors tend to average out. That’s why wearables are most useful for spotting your own trends against your own baseline, not for judging one night in isolation.

Compare yourself to yourself

Devices use different sensors and different formulas, so the same person can get different scores from different brands. Comparing your number to a friend’s, or to an online average, tells you little. Comparing your number this month to your number last month, on the same device, tells you much more.

Scores are summaries, not verdicts

“Readiness”, “recovery” and similar scores roll several signals into one number using a formula you can’t see. They can be a helpful nudge. But if your score says you’re exhausted and you feel great (or the reverse), don’t automatically trust the score over yourself. Look at the underlying data, and at how you actually feel and perform.

Not a medical device

Most wearable features are designed for general wellness, not diagnosis. A few devices have specific features cleared by health regulators, but a typical sleep or readiness score isn’t one of them. If a reading worries you, or something feels wrong regardless of what the device says, talk to a qualified professional.

Getting better data

• Wear it the same way, on the same spot, every day.
• Keep it charged so you don’t lose nights.
• Keep firmware updated, but note when a big update lands; it can shift your numbers.
• Don’t switch devices in the middle of testing a change.
• Pair the data with a quick daily note on how you feel.

The checklist

• I know which numbers are measured and which are estimated
• I’m comparing trends, not single readings
• I’m comparing myself to my own baseline, not other people
• I check the data behind any score that surprises me
• I’m wearing it consistently and keeping it charged
• I’m not switching devices mid-test

Next up: of everything a wearable tracks, sleep tends to move everything else. Sleep: the basics that move everything else covers where to start.