What is Heart Rate Variability (HRV)?

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 health. 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.
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For those using Visible’s Free Research App we use the phone camera to measure heart rate and HRV. The extraction of heart rate variability (HRV) from camera sensor data is known as photoplethysmography (PPG) and has been studied and researched extensively.
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PPG is an optical method used to measure small blood volume change in body tissues, such as the finger. As the heart pumps blood, the volume of blood in the arteries and capillaries changes by a small amount in sync with your heartbeat. This change leads to small changes in skin color.
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By placing your finger on the camera lens, the camera sensor is able to pick up small changes in skin color, and therefore determine the timing of the intervals between your heartbeats.

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.

Think of this as a three step process that turns messy heartbeat data into a clean, easy-to-understand score. The app converts raw heartbeat data into a 0-100 HRV score through three mathematical transformations.

​What it measures: The "jitteriness" or variation in beat-to-beat intervals. Your heart doesn't beat like a metronome - there are tiny variations in timing between beats, even at rest. This variation is actually healthy and shows your nervous system is responsive.

​How it works: Takes consecutive heartbeat intervals, calculates differences between them, squares the differences (removes negatives), averages them, then takes the square root.

​Why it matters: RMSSD is the established industry standard for detecting short-term autonomic nervous system activity - the part that controls involuntary functions like breathing and digestion.

RMSSD values don't increase linearly; they behave exponentially. Think of how earthquakes work. A magnitude 6 earthquake sounds like it’s twice the strength of a magnitude 3 earthquake, but it’s actually much stronger. It can be confusing when the numbers don’t communicate values like we might expect.

Natural log transformation shrinks RMSSD results down with numbers and spacing that make more sense in the context of Visible. These numbers reflect improvements to HRV in a way that is more realistic and manageable.

We then scale up the (ln)RMSSD range of 0-6.5 into 0-100 to create a simple score that is easy to understand, both day to day and when comparing long term trends.

​Formula: HRV Score = (ln(RMSSD) ÷ 6.5) × 100
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These transformations mean that the HRV value you see in Visible might be different from 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. We suggest doing it before you've got out of bed, to get as close to a resting HR as possible. 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 suggestion is informed by the expertise of Visible’s advisors and HRV experts. They say that this is the best way to establish a baseline that can be compared on different days.

Heart rate variability is based on individual circumstances. Think of comparing your HRV to someone else's like comparing heights, when you don't know their age - you can't judge your height against theirs when you don't know anything about them!

Many factors such as the nature of your illness and its severity, your recent activity levels and even your emotional state can affect it. Your own HRV can vary a lot day to day, so trying to compare one number to another isn't always the best way to understand your data and improve your pacing.

For this reason we always emphasize looking at long-term trends in HRV over comparing yourself to others.

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