A few days ago I bought a Fitbit Charge. A colleague had praised its heart rate monitor’s accuracy and I wondered about my own heart rate when running up hills.

According to the Charge, my heart rate goes up and down more than I’d previously realized. A lot more.

I’d always pictured my heart tracking along a step-table of steady states, the rhythm moving up or down in minutes-long blocks… 65 beats per minute when sitting or moving slowly, 100ish when walking quickly or doing a task, 130 when running and 140 when sprinting up a hill.

Here’s the reality (give or take a few beats) as tracked by the Charge this morning: Sitting reading a book, my heart rate is at 56… 58… 57 BPM. Straighten up, and I’m at 64 or 65. Stand up to get coffee, and I’m immediately at 73, then 74… 75… 72. Bend over to pick a newspaper and toss it the recycling… zoom up to 90 then glide down to 84. Sit down and within 4 seconds my rate is 65, then in a few more beats at 58… 56… 57.

This graph’s five minute averages only hint at the second-by-second roller coaster ride.

The gyrations are not a glitch or artifact of the device’s own mechanical noise. I know this because my Charge is fine-tuned enough to detect tiny heart rate variations that mirror my breathing. (The Charge relies on the fact that your heart rate slows slightly on out-breaths. The fitter (or happier) you are, the bigger this fluctuation.)

In my pre-Charge life, running was the only activity that regularly made me ponder my heart rate — my chest sometimes pounds when I sprint up hills. Ironically, the Charge shows that running is the activity in which my heart rate is most predictable. My heart goes to 150-154 BPM within the first dozen steps of a run and then, depending on hills, stays between 145 and 168 throughout.

Big or small, all this flux happens below the calm surface of human consciousness. It’s not just that you don’t notice this constant motion or somehow ‘miss’ it. The brain, in fact, is engineered to suppress any awareness of most of the body’s constant labor — its gurgling, bellowing machinations as well as its white-gloved recalibrations. Without diligent counter-programming by your brain, the whoosh and thump of each heartbeat would drown out all other sounds and swamp you with anxiety. (The brain executes a similar double trick with vision — twitching your eyes constantly to keep cones and rods firing, while also suppressing any awareness of that twitching.) Without the ability to filter out the constant buzz and glitter of existence, you’d be overwhelmed.

Humans rely on two feats of engineering, each astonishing in its own right. First, the body is endowed with an ability to constantly and instantly adjust to changing conditions. And, second, the body has an equally constant and instantaneous capacity to suppress awareness of those adjustments.

Until you buy a Fitbit Charge, that is.

(Update: more testing and heart rate results from two different types of runs.)