Your respiratory rate, often referred to as your breathing rate, is the number of breaths you take per minute. For most people, average breaths per minute typically range from 12 to 20 while in a state of rest.
Each breath, or respiration, has two phases, inhalation and exhalation. Oxygen is brought into your lungs during inhalation, and transported throughout your body in the bloodstream. Carbon dioxide is then eliminated and dispersed from your lungs during exhalation.
WHOOP measures respiratory rate during sleep and reports it in units of “respirations per minute,” or RPM. The number you see displayed in the app is your median number of RPM over the course of the night while you are sleeping.
As you can see above, the majority of WHOOP members have an average respiratory rate that falls somewhere between 13 and 18 breaths per minute.
WHOOP calculates respiratory rate from your raw heart rate data by taking advantage of a phenomenon called Respiratory Sinus Arrhythmia.
Here’s how it works: When you breathe in your heart rate increases, and when you breathe out it decreases. This allows your body to preferentially pass blood by the lungs while they are full of oxygen. Because the autonomic nervous systems increase heart rate during inhalation and decrease it during exhalation, we can see respiratory rate in your continuous heart rate data by looking for this cyclical pattern of increasing and decreasing.
WHOOP is the first wrist-worn wearable device to have the accuracy of its respiratory rate measurement during sleep validated by a third party. In a study published in the Journal of Clinical Sleep Medicine, WHOOP respiratory rate was shown to be within a single breath per minute of gold-standard truth.
WHOOP has actually been tracking respiratory rate throughout sleeps for a long time now. We use minute-by-minute alterations in respiratory rate in our sleep staging algorithm because respiratory rate predictably changes slightly during different sleep stages.
Initially, WHOOP didn’t display your average respiratory rate because it generally follows a similar trend as heart rate variability (HRV) and resting heart rate (RHR), so we weren’t sure how our members could action this information.
However, in an effort to always improve the analysis and recommendations we provide, we discovered something interesting: While it is true that respiratory rate is generally an indicator of cardiovascular fitness and load and therefore increases when RHR increases and decreases when RHR decreases, it is also a remarkably stable metric. From night to night, you should not expect to see much change in your respiratory rate statistic. But, when it does change that change tends to be meaningful.
While HRV and resting heart rate may meaninglessly change from day to day, respiratory rate generally does not. In statistics, this is known as the “signal to noise ratio,” and it determines how much variance you need to confidently conclude that a change is significant as opposed to random. Median respiratory rate has an extremely high signal to noise ratio, which makes it very easy to interpret and trust. Because of this, respiratory rate is useful for spotting sleeps in which something is off.
In my own data, for example, over a recent 30-day span my respiratory rate ranged from 14-15 breaths per minute every night except one–when I was sleeping in a middle seat on a red-eye plane from Boston to Reykjavik. My respiratory rate was 17 that night, and I slept terribly.