Why Your Garmin Shows a Heartbeat When You're Not Wearing It

Day 17: The Optical Heart Rate Illusion

"Why does your Garmin still record a heartbeat when you're not wearing it?"

You take off your Garmin, set it face-up on your desk, and glance at the heart rate reading. It shows 62 BPM. But you're not wearing it. What's going on?

How optical heart rate works

Garmin (like all smartwatches) measures heart rate using a photoplethysmography (PPG) sensor. Here's how:

1. Green LEDs shine light through your skin
2. Blood absorbs green light more than surrounding tissue
3. As your heart pumps, blood volume in your capillaries changes
4. A photodetector measures how much light bounces back — and the rhythm of those changes = your heart rate

Why it reads a "heartbeat" on a table

The sensor doesn't actually know what it's on. When placed on any surface — table, couch, skin, or carpet — the green LED bounces light back at some frequency. The sensor interprets subtle variations in that reflected light as a "pulse."

Wood grain, fabric texture, and room vibrations (from fans, traffic, your breathing nearby) all create just enough variation to fool the basic detection algorithm.

What to do when not wearing it

Place the watch face-down or on its side. This blocks the LEDs from pointing at any surface, stopping false readings and saving battery (the sensor turns off when it detects nothing reliable).