Take your watch off at night and look at the back of it.
Two or three tiny green lights are still blinking in the dark.
They have been doing that against your skin all day.
That is how your watch claims to know your heart rate.
It works beautifully in some moments and badly in others.
Knowing which is which changes how you read it.
How can a light read your heartbeat?
Blood absorbs green light more than skin and bone do.
Every heartbeat pushes a little more blood through your wrist.
So slightly less green light bounces back at that instant.
A sensor measures those tiny dips in reflected light.
Count the dips over a minute and you have a pulse.
The proper name for this is photoplethysmography.
It simply means measuring blood flow with light.
Notice what it is not doing.
It reads nothing electrical from your heart.
When is the reading dependable?
When you sit still and warm, with the strap snug.
Resting readings taken while you are calm are the best a wrist can manage.
Steady walking is usually fine as well.
Sleep readings tend to be good, because your arm is not swinging.
In those moments the watch is close to a finger on your pulse.
When does it go wrong?
When you move suddenly, sweat heavily, or wear the strap loose.
Every jolt lets stray light in.
That is why the number lags during weights, sprints or a cricket match.
Cold hands make it worse, since less blood reaches your wrist.
Dark tattoos over the sensor can block the light entirely.
If a workout number looks impossible, it usually is.
Why is resting heart rate the useful one?
Because it is measured in the conditions the sensor handles best.
It is your pulse when you are awake, calm and still.
Watched over weeks, it quietly tracks your fitness, sleep and stress.
A slow fall over months usually means your fitness is improving.
A rise lasting several days can mean illness, exhaustion or too little water.
That is a reason to rest and pay attention.
It is not a reason to panic.
Things to Remember
- Keep the strap snug and the sensor window clean.
- Trust resting and sleeping readings over workout readings.
- Expect wrong numbers during weights, sprints and cold weather.
- Watch your resting heart rate over weeks, not hours.
- See a doctor about a lasting change, never a single reading.