A question that occasionally comes up is whether you could use heart rate variability (HRV) continuously, during the day, to identify mental stress with e.g. a wearable
In my opinion, the short answer is no
Here, truly less is more
For the long answer, see below 🧵👇
Normally, when we talk about stress, we consider all sources (mental and physical), as physiologically, there is ~no difference (see slides 23-24 here: slideshare.net/marcoalt/heart…)
This means that you cannot separate mental stress from physical stress in HRV, in the "real world"
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This is also why HRV measurements should be taken in certain well-defined conditions, for example, first thing in the morning or during the night, so that baseline physiological stress can be captured in a repeatable context (more details here: medium.com/@altini_marco/…)
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During the day, many transient stressors will play a role, from diet to light exercise to mental stressors, etc.
Many of these transient stressors will have no long-term impact, while others will pile up and impact your baseline physiology
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Thus, morning or night measurements will be highlighting periods of high stress (both mental and physical) in response to strong acute stressors, or chronic ones, as shown here:
While some devices try to use daytime data to look into stress, the nature of what is being measured (i.e. the autonomic nervous system activity) makes it so that in my opinion, the limitations are huge and practical applicability very limited
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For example, you could think of building a model in which your own HR / HRV during the day is compared only to your own history at the same time of the day (to account for individual variability and the effect of the circadian rhythm), and also only when you are at rest ....
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... however HR / HRV will react dynamically to everything: the time of your lunch, or the macronutrients in there, or if you had a hard workout, or were just chatting with a friend, your HR will also be high and HRV suppressed, without any movement or 'dangerous' stressor
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Plus, stress can be good. The goal should not be to get warned about any physiological change that resembles stress
Some stressors have a "negative" acute impact on the body but then lead to improved health (e.g. exercise)
Continuous measurement trivializes these aspects
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My point is that these metrics are useful when properly contextualized, e.g. morning or night measurements or even just before / after a stressor, during aerobic exercise, etc., but when looking more broadly at daytime data (automatically) with consumer wearables, not really
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Note that certainly larger stressors can be identified (e.g. sickness), but there would be too many false positives (periods in which you are not actually stressed), for this to be effective, in my opinion
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HRV is insightful in the context of mental stress of course, but continuous measurement and analysis without overinterpreting any minor change is a hardly solvable challenge unless the user is constantly in the loop (which would be alienating, talking about mental health..)
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Less measurements, more insights
One data point per day is really all you need to assess physiological stress and do something about it, whenever possible
Plus, this approach leads to a healthier relationship with technology, wrt continuous measurement (in my opinion)
/end
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I measure my resting physiology in two ways, which I consider equivalent (with some nuances):
• Morning measurement: 1 minute using the phone camera and HRV4Training
• Night data: average of the full night using the Oura ring (which I also pull in HRV4Training)
A 🧵👇
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I use both for obvious reasons (see bio), and have discussed previously in more detail night data: medium.com/@altini_marco/… - covering also the main differences with morning measurements
I always like to back up the theory with real-life data, so here are 6 months of data
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The data I am showing is the same I had shown a few days ago
I want to highlight how the data is extremely similar when there are no disruptions in heart rhythm, and how depending on your preference or constraints, you can pick whichever tool
Had mixed feelings at the beginning as we had an issue on iPhone 8, which was fixed for HRV4Training, but not for Camera HRV (showing poor results). This is most likely reflected in the reported quality though
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another important point is that at this point it should be clear that PPG vs ECG is not what the conversation should be about. PPG works great and can easily outperform ECG (see Firstbeat + chest strap ranking poorly)
Why? Because HRV is all about how you handle artifacts
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