Heart Rate Variability (HRV) ) is a measurement of the variation in time between your heartbeats. It can be easy to confuse Heart Rate Variability (HRV) with Heart Rate (HR). Heart Rate measures the average beats per minute, whereas HRV measures the change in time (or variability) between successive heartbeats.
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To understand HRV, it’s important to know that the human heart is not a metronome. A heart rate of 60 beats per minute suggests one beat per second; in reality, there are millisecond variations between successive heartbeats. Some beats are more like 0.9 seconds apart while others are more like 1.2 seconds apart.

HRV is an important metric because it reflects the balance between your sympathetic nervous system (which activates stress responses) and your parasympathetic nervous system (which promotes relaxation and recovery). More than 55,000 studies show that it reflects the activity of your body’s stress response and recovery systems.

Research has shown that HRV is lower in people living with both Long Covid and ME/CFS. For this reason, you might find it helpful to track over time as an indicator of your wellbeing. Generally speaking, a trend towards increasing HRV is a good signal of improving overall health.
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Tracking HRV may also help you to better manage your illness. Measuring it as part of the morning check-in creates a powerful feedback loop that can help you understand what changes (whether it’s pacing, medication or something else) might be improving your health.

To calculate HRV from your heartbeats we apply a Root Mean Square of Successive Differences (“RMSSD”) calculation to the beat-to-beat intervals. RMSSD is the industry standard time measurement for detecting Autonomic Nervous System activity in short-term measurements.

Next, we apply a natural log (ln) to the RMSSD values. RMSSD values do not chart in a linear fashion, so it can be difficult to make sense of the magnitude of changes in RMSSD as it rises and falls. By applying a natural log (ln) to RMSSD, we can produce a number that is more linearly distributed.

Finally, we expand the ln(RMSSD) to create a 0 to 100 score, which is the one you’ll see in the Visible app. The ln(RMSSD) value typically ranges from 0 to 6.5. Using over 6,000,000 readings from the EliteHRV database, we have been able to sift out anomalous readings and create a more precise scale where every reading fits in a larger range.

This means that the value you see in Visible might be slightly different to the one you see in other apps. But you should find that over time, the 0 to 100 scale makes it easier to compare your HRV between days, and spot trends in your HRV.

We currently only recommend measuring HRV once a day in the morning. Our aim with Visible is that you can measure your HRV once a day so you can track trends in your illnesses over a longer period of time rather than checking in multiple times a day to analyse the impact of individual events. 
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First thing in the morning is the most reliable way to track trends in your HRV over long periods of time. HRV is a particularly sensitive metric and being in a repeatable environment with minimal external stressor is important for comparable day-to-day readings.
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This is in line with discussions we have had with our advisors and studying the work of other experts on HRV who have said that this is the best way to establish a baseline that can be compared on different days.
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