S1E13: How is Stress measured by a Fitness watch?
I like listening to the Huberman Lab podcast, hosted by a Neuroscience professor from Stanford. In one of his episodes, he spoke about managing stress, anxiety, and the tools for the same. I learned several practical tips for managing stress in real time but I was curious to know if we can somehow quantify stress and track them using existing gadgets. This was when I came across the EDA scan app feature in Fitbit while exploring options for the same. But before we jump in to distill how it works, I’d like to share some facts about the human body as it can be easier to follow later.
Introduction
We have sweat glands that are found throughout our skin and they produce sweat when we are stressed, excited, or when we exercise a lot. The more stressed we are, the more sweat is produced by our glands as we do not consciously control sweat. It’s basically an auto-response originating from our sympathetic nervous system. Even if we can’t see with our naked eyes, it happens at a micro level.
And we know that every object can naturally be characterized by electrical resistance and conductance, including human skin. As sweat is comprised of salt water which can conduct electrical signals, the more sweat produced, the greater the flow of electricity. This means that skin conductance is high when we are stressed out. Fitbit aims to measure this response noninvasively aka EDA (Electrodermal activity) as a means to classify our level of stress. It tracks the changes in the electrical conductance of the skin. Note that, exercise can also cause a lot of sweat. So tracking your stress level when you’re not excited or exercising is suggested for getting a close-to-accurate response. Now, let’s look at the hardware design.
Hardware Setup
Electrode: An electric conductor that carries current into the non-metallic part of a circuit like some liquids, gases, etc.
The rim around the display of the fitness watch is partitioned into different sections. The shiny rounded rectangle on the side of the watch shown here is an electrode on one side of the watch. Similarly, it has an electrode on the other side as well. The rest of the housing on the sides is made out of non-conductive material like plastic, etc. Note that the location and the design of the electrodes could change from model to model.
Figure 1: Fitbit with electrodes on the sides
Method of Usage
Fitbit has an EDA Scan App that is used to measure electrodermal activity. So here is how it’s used. You could place your thumb and index finger on the left and right side of the watch respectively, where the electrodes are positioned, and wait for a few minutes. During this time, it measures the conductance of your skin and the pulse rate change. Finally, it asks you to classify your mood into stressed, neutral, etc. It considers that to provide data on how stressed you are.
Working Principle
As the electrodes are electrically isolated, you close a circuit when your fingers are on the electrodes. Then, a small fixed current is injected across the electrodes which flow through the skin. Now, the voltage across the electrodes can be measured. Thus with the voltage and current values available, we can compute your skin conductance using Ohm’s Law and the fact that Conductance = 1/Resistance.
Conclusion
The skin conductance value measured and mapped to a stress level over a month every day can show you how the stress level has changed over time. The App also recommends you perform breathing exercises, meditation, etc., to reduce stress. And this is how Fitbit detects and tracks your stress levels.
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