HRV results vary according to how and when they are measured.
The “gold standard” for clinical HRV assessment is the 24 h HRV recordingAn Overview of HRV Metrics and Norms with the subject linked to an ECG machine in a strictly controlled environment. These recordings achieve greater predictive power than short-term measurements. Although the mathematical formulas are the same, the results for 24 h and short-term HRV measurements cannot substitute for each other and their physiological meaning can profoundly differ.
Results from wearable devices, although they may be 24 h recordings, are also not a substitute for recordings taken under clinical conditions.
The results from short term (~ 5 mins) and ultra short term (< 5 mins) readings, when using the appropriate formulas, and taken under consistent conditions are reliable While having limited predictive capabilities, they are able to provide feedback that gives an accurate assessment of the subject’s health and stress levels.
There are many factors that influence the HRV reading. Inhaling causes the pulse rate to increase, and exhaling slows the heart down again. Agitation, stress, movement, and stimulants all cause an increase in heart rate.
For the most accurate results, we recommend that users of the HRV Health platform take their readings first thing in the morning, in a comfortable position and in a calm state, breathing normally. Our tests confirm that reading or writing emails, watching the news, or holding a conversation can all affect the reliability of the readings, adversely influencing the outcome.
Waking up, your head buzzing with everything that needs to get done, and knowing that you don’t have enough time for everything, seems a crazy time to steal three minutes to take the HRV reading.
But that’s the time when you need it most. Your stress levels will be clear from the reading, and that is incredibly helpful.
Our users have discovered that the stress index can be a warning that they need to take action, rewarding themselves with the pleasures that bring relief. And that relief shows up in the numbers the next morning. (Note: a late night with alcohol will also show up in the numbers, and not in a good way.)
The stress index measures both mental stress and physiological stress. If you did a hard workout the night before, a high stress level is probably symptomatic of that. If you had an argument with your partner, then the stress figure will be high. The stress indicator has proven to be a reliable metric. Use it well.
HRV changes throughout the day. That’s why we recommend taking the reading once per day, preferably first thing in the morning. But, if you miss the morning reading, or prefer taking it at some other time, doing the reading in a calm relaxed state, at least 60 minutes after eating, will provide in a reasonably accurate result. The important thing is that you are completely relaxed, and your resting heart rate is accurate.
Many of the HRV Health Platform are athletes. They are helping us with an experiment. They take a reading immediately after their workout, and then a second reading one hour later. The immediate reading reflects the stress of the effort, with single digit RMSSD results. The readings after an hour are surprising, often better than the reading taken in the morning. It proves their powers of recovery, their levels of fitness, and how good exercise is.
HRV is very personal. We recommend that users experiment and find what works best for them. Start with the early morning reading to establish a baseline, and then experiment to find what works best for you.
Our data confirm that this approach achieves an excellent balance of convenience and accuracy, producing figures that you can rely on in your quest for good health and quality of life.