Tuned In, Stressed Out: Measuring and Managing Stress with Physical Indicators

Hans Selye, the father of stress research and a pioneer in the field of endocrinology once said, “It is not stress that kills us, it is our reaction to it.” 

Whether it’s mental stress reactions like anxiety and panic or withdrawal and apathy, or physical stress reactions like increased heart rate and sweat levels, managing stress requires a trained sense of mindfulness. And while some stress is good, like a motivating work deadline, most stress can be taxing. Paired with the global pandemic and general uncertainties about the future, stress is ultimately unavoidable. But regardless of stress type, as Hans Selye pointed out, in order to live long, healthy lives, we need to monitor how we perceive, react to, and manage stress.

But how do you know when you’re stressed? Well, the Fitbit research team set out to explore if wearables can measure physical stress responses to give users a better understanding of how they can monitor and manage stress in their lives. They also wanted to see if factors like age, gender, and activity had an impact on both physical stress metrics and perceived stress levels. 

The Study 

First, our research team asked, “Can we measure a psychological state (like stress) with physical indicators on Fitbit?” 

It’s a challenging question with a spectrum of variables ranging from physical health, living situation, and varying cultural norms associated with stress management. But, whether we’re aware of it or not, each time we’re faced with a stressor, our bodies undergo physical and mental changes in an attempt to maintain balance. Scientifically speaking, stress is the relationship between your body and a situation that you appraise as unmanageable or threatening. Your appraisal of the situation (or stressor) determines your stress response, which can manifest in a variety of physical reactions ranging from a racing heart beat to sweaty palms. 

When you appraise a situation as a stressor, your sympathetic nervous system releases stress hormones, like adrenaline and cortisol, to prepare you for a “fight or flight” response. This process can cause all sorts of changes in your body, some of which can be measured with wearables via metrics like heart rate variability, or HRV, and electrodermal activity, or EDA, which is measured with a new sensor on Fitbit Sense.

Due to the complexity of stress variables, our team developed a Stress Management Score which combines metrics including sleep patterns, exertion (steps, activity), heart rate and heart rate variability (HRV), and EDA to give users a daily score about how their body is reacting to stress. 

Over the course of four weeks, approximately 4,000 participants received their daily Stress Management Score and were asked to take two daily surveys: a daily morning survey asked how ready participants were to take on the day on a 1-10 scale, and similarly, a nightly survey asked participants to reflect and ask how ready they were to take on the challenges of the day. We also gave participants a separate survey to measure their perceived stress over the last month, called the Perceived Stress Scale (or PSS), once at the beginning and again at the end of the study.

Our Findings

It turns out that these physical, measurable indicators that make up the Stress Management Score correlate with perceived feelings of stress. We saw that the average Stress Management Score of a participant over the four week study significantly correlated with the PSS. Additionally, participants’ mean Stress Scores for the month significantly correlated with the mean daily and nightly survey scores. 

We also saw some interesting differences in stress across demographics and activity levels. Everyone perceives and reacts to stress differently, but there are statistically significant variances across age, gender, and activity: 

• Older participants report less perceived stress, and people in their 20s and 30s report the highest perceived stress levels. 
• Female participants report slightly higher perceived stress levels than do men. 
• The more active you are, the less perceived stress you report. The more steps and/or Active Zone Minutes you acquire each day, the lower your perceived stress score. 

Conclusion 

Using these results, Fitbit further developed its Stress Management Score, available on the new Fitbit Sense. The score looks at some of these metrics to provide users with a daily calculation of their physical stress level based on three components: 

1. Exertion balance: Gauges the impact of activity like exercise and steps
2. Responsiveness: Assesses your fight or flight response through metrics like heart rate, HRV and EDA 
3. Sleep patterns: Includes metrics that measure the quality and quantity of sleep 

This score was designed in part to help people plan their day. For example, if you receive a high score—which is a good thing—it means your body is showing few signs of physical stress, so you may consider taking on a new project or exercising. If your score is low—which is not ideal—it means your body is showing signs of stress, so you may want to give yourself a break—go to bed early or meditate. Over time, your score can also reveal connections between your stress level and key health metrics like activity and sleep.

With this score and other mindfulness tools from Fitbit, Fitbit users can learn about the impact of stress on their body, track their body’s reaction to stress, and gain a deeper understanding of their body to help guide them to a path to better health and wellness.