How is the stress metric calculated? Is it reliable?
Based on my watch, I seem to be highly stressed throughout the day.
How is the stress metric calculated? Is it reliable?
Based on my watch, I seem to be highly stressed throughout the day.
You can think of stress level as the inverse of Heart Rate Variability (HRV). HRV is the variations between consecutive heart beats. I once heard this explanation about HRV that really stuck with me (source: Sarah Does Sass: Your heart's secret language: HRV explained):
The heart has a push and pull mode of delivering energy to the rest of the body. When it senses that we might need the energy any moment, like in a stressful situation or during activities, it beats very evenly, it pushes the blood through without much feedback.
When we are more chill, the nervous system can signal when it needs more energy/oxygen, so the heart beats become less regular: it's happening in a more on-demand basis.
So the watch establishes your base HRV value the more you wear it (you can see those ranges in the HRV Status widget). Of course people would be confused by an ever changing number in miliseconds, so Garmin came up with this stress level.
When your HRV is high, then your stress level is very low (close to zero); when your HRV is almost 0, then your stress level is close to 100.
So this way you can understand a bit better how the stress level graph works... it doesn't necessarily mean that you're mentally stressed. It can be that your body is requiring more energy. My stress level spikes and slowly (over hours) decreases after workouts, even after the part that period that Garmin blanks out to avoid confusing people, as we would always have super high stress right after workouts.
If you did some effort, the body needs to work on recovery, which is stress, even if it's the healthy kind of stress. But stress is stress, it does tire us.
Here are two more sources that more or less confim my theory about the relation between HRV and stress:
The Garmin article is a lot less clear about the actual relation than the first article. I formed these theories after paying close attention to my stress levels in the last 8 years, when I started wearing the Forerunner 935, and now the Enduro 3 for 2 years. Hope this helps clear it up a bit. 
