1. What exactly are we measuring?
Studying the heart provides us with a vast amount of information about our bodies. From the beat to beat, heart rate is constantly changing to meet the needs of life. Heart rate variability (HRV) is the variation in time between heartbeats. HRV is universally accepted as a non-invasive marker of autonomic nervous system (ANS) activity – which response to everything: how you exercise, sleep, perceive stress, and respond to emotions.
While basic heart rate (HR) counts the number of heartbeats per minute, HRV shows the exact change in the time between successive heartbeats. It is also called RR intervals or inter-beat intervals.
Stress Monitor doesn't provide medical advice and isn't a substitute for a healthcare professional. If you have any questions or concerns about your health you should immediately contact your physician.
HRV overview: An Overview of Heart Rate Variability Metrics and Norms
Correlation between stress and HRV (60 adults participated in this study): Short-Term Effect of Stress on Heart Rate Variability
Correlation between stress and HRV (28 adults participated in this study): Influence of Mental Stress on Heart Rate and Heart Rate Variability
Review of science literature that proofes correlation of HRV and stress (considered 37 publications on the topic) : Stress and Heart Rate Variability: A Meta-Analysis and Review of the Literature
Correlation between emotions and HRV: Artificial neural networks-based classification of emotions using wristband heart rate monitor data
Emotions and HRV (27 adults participated in this study): Recognizing Emotions Induced by Affective Sounds through Heart Rate Variability
2. Why are Apple Watch measurements reliable and how does it work?
In a recent study for validation of the Apple Watch for HRV measurements, it was confirmed that the Apple Watch could be a reliable source of data for HRV calculations.
As we mentioned above, Stress Monitor calculates your stress using heart data that your Apple Watch records during the day using the Apple Watch sensors. The number of records of your R-R intervals depends on how active you have been during the day, as some heart data recorded only in the state of rest. On average it's 3-6 measurements during the day.
The app doesn’t write any data to Apple Health, it only reads and processes the available data from Apple Health. To have this data in the Apple Health app you should have Apple Watch.
As well, it's possible to indirectly influence the amount of beat-to-beat measurements that you have in your Apple Health App. You can do a Breathe session in Apple Watch App - Breathe (inbuilt/standard Apple Watch app). Stress Monitor doesn't have any integration with the app Breathe, it's just a way to have updated beat-to-beat data in your Apple Health App. Right after the session of Breathe, new data of beat-to-beat measurements became available in Apple Health App. Stress Monitor would provide you with the new calculation of stress in the app. It's not required to do Breathe - just in case if you want to increase the number of stress measurements.