Someone in your family wears a fitness band.
It tells them they slept badly, or that they did not walk enough.
Most of us take those numbers at face value.
But a band does not measure most of what it shows you.
It measures two or three things and works out the rest.
What is actually under the strap?
Turn the band over and look at the small glass window.
Behind it sit a few lights and chips the size of a grain of rice.
The green lights are the optical heart sensor.
They shine light into your skin and read what bounces back.
Beside them sits an accelerometer.
That is a chip which senses movement and which way is down.
Costlier models add a skin temperature sensor.
They also add a red or infrared light, used for oxygen readings.
Nothing there goes below your skin.
What does it measure directly?
Movement is the one thing a band genuinely measures.
The accelerometer feels every swing and jolt of your arm.
Pulse is the second, read as blood moving just under the skin.
If your band has GPS, it also measures your position from satellites.
Everything on that short list is a real physical reading.
What is it only guessing at?
Steps are a guess.
Software studies the movement data and decides that pattern was a step.
Calories burned are a much bigger guess.
They are built from your age, weight, height and pulse.
Distance without GPS comes from your steps and an assumed stride length.
Sleep stages are inferred from how still you lie.
Stress scores usually come from the variation between heartbeats.
These are calculations, not measurements.
Why does that difference matter?
Because a measurement stays roughly true.
A calculation drifts with every wrong assumption behind it.
A pulse taken while you sit still is close to correct.
A calorie figure could be far off and you would never know.
So judge your day by the measured things.
Use the estimated ones only against yourself.
Your sleep score today against last week is useful.
Your sleep score against your friend's is not.
Things to Remember
- Treat pulse, movement and GPS distance as genuine measurements.
- Treat calories, sleep stages and stress scores as estimates.
- Compare a number only with your own earlier numbers.
- Watch the direction over weeks, not the daily wobble.
- Never make a health decision on a band reading alone.