How does Heartify Work?

Why monitor your heart rate?

Your heart rate is not just about the number of beats per minute. That is because your heart rate is affected by the work of your nervous system which reflects the state of your body as a whole, offering a wide diagnostic potential. Decades of scientific research have shown that this data can be applied anywhere from monitoring your heart health by tracking important symptoms, optimizing exercise load, and preventing overexertion from the stressors of daily life way before it becomes a serious problem. All of this can be done with the help of heart rate variability.

What is heart rate variability?

Heart Rate Variability (HRV) refers to the tiny, almost imperceptible differences in the intervals between each successive heartbeat. By analyzing this data, a number of scientifically studied parameters can be calculated, offering deeper insights into your health and well-being. This is crucial because your heartbeat is not solely controlled by the heart muscle itself; it is also influenced by your autonomic nervous system, which has two primary components.

The sympathetic nervous system (SNS) is your body’s response to stress, controlling your fight-or-flight mechanism by activating additional resources and typically increasing your heart rate. In contrast, the parasympathetic nervous system (PSNS) is responsible for rest and digestion, working to slow your heart rate. Both of these systems constantly react to changes in your body and environment, such as sleep patterns, exercise, meals, illness, or even your mental state.

By tracking HRV, you can monitor how these systems are functioning and responding, as each HRV parameter has been scientifically linked to specific health indicators. This makes HRV a valuable tool for proactively keeping tabs on your overall health. However, keep in mind that while there are significant amounts of research on this topic, the exact cause-and-effect connections between HRV metrics and your physical state are not always straightforward.

How do measurements work?

Heartify can measure your heart rate and HRV using two main methods: using your phone’s camera or by connecting an Apple Watch for automatic tracking.

Camera measurements rely on a well-tested technique called photoplethysmography (PPG), which is used un many clinical devices such as pulse oximeters. It works by shining a bright light, like your camera’s flash, onto your skin and recording the subtle changes in light intensity, as oxygenated blood flows through the capillaries in your finger with each heartbeat [1]. This method works because our skin is slightly transparent – you can even sometimes see those blood vessels yourself, and research has shown that PPG is effective for people of any skin color.

Don’t worry if you get a false positive by trying it on inanimate objects: our algorithms are precisely adjusted to work on real patterns of the human pulse.

If you own an Apple Watch, you can connect it to Heartify to receive automatic monitoring throughout the day, and you can also use the Breathe feature of the built-in Mindfulness app to trigger a measure manually when you have a couple minutes for a measurement. Apple Watch sensors use roughly the same technology, but they tend to be slightly more accurate since they have more dedicated heart rate sensors rather than just the one in your camera.

The following is a distribution of pulse measurements between the Apple watch and camera recordings. The similar shape of the distribution chart indicates that both measurement methods provide very close results in terms of pulse measurement accuracy.

To take accurate measurements, make sure to follow the instructions at the start of the measurement. Place your index finger onto the camera: if you have multiple back cameras on your phone, we will indicate which one is the best to use. Take a look at the viewfinder on the screen: it should turn completely red if you have placed your finger correctly. During the measurement, sit in a relaxed position and avoid any movement or even talking. Moving and speaking influence your breathing, which in turn affects your results. Keep measuring until the end to get all of your detailed results.

The best time to measure HRV is usually in the morning, before breakfast or any strenuous activity. Exercise and meals activate the sympathetic and parasympathetic nervous systems, respectively, and can therefore also affect your result.

How do you calculate things like stress and energy?

First, it’s important to understand that HRV is not just a single parameter. There are several primary parameters that are usually analyzed in research, such as SDNN, RMSSD, pNN50, and others. These are usually mathematical derivations, such as calculating the standard deviation of the distance between each heartbeat peak (SDNN), or by taking the average of the square of successive differences between intervals and then getting a square root of the result.

Also, there are a number of frequency-domain parameters that are obtained by breaking down the raw HRV data into different frequency components, resulting in low-frequency (LF), high-frequency (HF) and very low frequency (VLF) bands. Currently, it is considered that the LF band can indicate physiological stress and sympathetic activity, while the HF band denotes activity of the parasympathetic systems. The VLF band is less understood, but there are some hypotheses connecting it to circadian rhythms, thermoregulation, and cognitive focus [2] [3].

You can learn more details about how all these metrics are calculated from this research article on PubMed: An Overview of Heart Rate Variability Metrics and Norms [4].

There is no scientific consensus about one single measurement of HRV. Different devices and apps choose which metric to use as the primary one. For example, Apple Health uses SDNN.

All these measurements have been extensively studied in various groups of people, analyzing the correlations between each metric and symptoms of general and cardiovascular health, sports performance, mental states, fatigue, physiological stress, and more.

Heartify gives you all of these parameters and the ability to track their changes over time. In addition, because these metrics can be hard to interpret, Heartify also introduces some extra metrics to help you keep track of your health and well-being.

Stress

When it comes to HRV, stress is not limited to your mental state. The sympathetic nervous system (SNS) triggers the physiological stress response whenever your body needs to activate additional resources. This is a very important evolutionary mechanism that has allowed our ancestors to survive using the fight-or-flight response, however, in modern times it can be caused by entirely different factors. Things like exercise, lack of sleep, environmental factors, extreme cognitive focus, and yes, even your emotional state, can trigger the stress response. The Stress metric is calculated based on the spectral analysis of frequency-domain metrics like LF and also uses Bayevsky’s stress index [5] [6].

When you get a high stress score on your measurements, consider what you have been doing in the last hour. Sometimes, you might feel fine in the current moment, but that can simply be because your body is working overtime to ensure normal functioning, and this can take a toll on you later if you don’t allow yourself to destress. Keep in mind that measuring after exercise can also show elevated stress: this does not mean that exercise is harming you! It only shows that your body has worked hard and needs some time to recharge.

Here is a curious chart showing the maximum stress levels during a 1-day period. Notice the gradual increase from morning to daytime when we are usually the busiest, and a decrease towards the evening when a lot of people are going to bed. The spike in the morning is most likely caused by the natural elevation of the stress hormone cortisol that helps our bodies wake up.

Energy

The Energy metric comes from the total activity of your nervous system by adding up all the frequency-domain metrics we mentioned above. Energy will usually be higher when both your sympathetic and parasympathetic nervous systems show elevated activity. This does not necessarily mean that you would feel more or less energetic in terms of getting enough sleep, although this can also affect your Energy measurement. Higher energy levels may indicate that you are ready for new challenges, whether because you are well-rested, or because your body is using up extra internal resources to maintain its functioning.

Source: Diurnal Heart Rate Variability Fluctuations in Normal Volunteers [7]

Tension

In Heartify, Tension is an umbrella metric that can show insights into your general state of health. For most people, this metric will generally stay the same unless your body is going through some tough times.

Measurement score

The main Score parameter is the cumulative rating of all your measurement parameters. It is not a straightforward average, but rather a weighted scoring that takes into account your pulse, Stress, Energy, and several other metrics. Therefore, if one of the metrics is very low, it might impact the overall score even if you feel relatively fine. This just means that you might need to take a look at the measurement results and try to find some time to take a brief rest.

HRV

As we’ve mentioned above, “HRV” in itself is not a single parameter: in the case of Heartify, we’ve chosen a mathematical derivative of RMSSD to represent heart rate variability in general, because it is one of the most well-studied metrics in research. Keep in mind that the HRV parameter in Apple Health uses SDNN for this purpose. Although there is also plenty of research on SDNN, it’s important to note that it is a very different metric. In general, HRV in itself is a very individualized parameter, so we recommend focusing on its dynamics, both throughout the day and across several days, rather than its absolute value.

Health Balance

The Health Balance chart can help you see how the balance of the two opposing parts of your autonomous nervous system have been changing with the most recent measurements. This metric is based on the relationship between LF and HF bands. Stress will be prevalent when your sympathetic nervous system is more active, and Rest may take over when your body is recovering or digesting food. Ideally, you should aim for a 50/50 balance between these two systems.

System interactions

This chart shows how the three frequency-based components of your HRV balance each other out, adjusted for age norms according to research. Ideally, these three components (LF, HF, and VLF) should each take up about a third of the chart. The more balanced they are, the higher this parameter will be. The relationship between these components can help discover which parts of your nervous system are engaged most and take measures based on that information.

Brain-Body Interaction

This chart is based on the power of the VLF band from your HRV measurement. According to some research, the very low frequency band can be responsible for the feedback processes between the heart, brain, and the spinal column.

Some studies have shown a relationship between low VLF power and a number of severe conditions like arrhythmia [8], PTSD [9], and inflammation [10].

Readiness

It probably sounds self-evident that low stress and high energy ensure that you are likely to feel and perform better throughout the day. Individual measurements may be impacted by your activities 1-2 hours prior, such as meals and light activity, so Heartify also offers an additional, less reactive metric to help you track your overall readiness for action, whether that is work, exercise, or just your daily tasks. Think of the Readiness score like your own personal battery. For optimal results, take a measurement first thing in the morning (or sync up your Apple Watch) and keep tracking your Readiness throughout the day to prevent yourself from overreaching.

Heart Health and Clinical Monitoring

Heartify and HRV tracking are great tools for at-home health monitoring, both in terms of scientific health parameters and in terms of maintaining a better physical and mental state. However, please keep in mind that they should not be used as diagnostic tools on their own, especially if you have a previous history of serious illnesses, especially heart disease or diabetes.

Box plot of SDNN distribution among groups with self-reported symptoms.

Licensed medical practices can offer many more diagnostic tools that can only be done in person, and lab tests are often the most decisive factors that can help identify certain conditions.

Here are some additional steps you can take to maintain and improve your health:

Schedule regular checkups with your doctor. If you don’t have specific concerns, you can start with your family doctor or GP, who can then refer you to relevant specialists.
As trite as it sounds, diet and exercise! There have been thousands of studies that had investigated their benefits, whether separately or in combination, all across various groups of people, and the results are overwhelmingly positive. The good news is, this doesn’t mean you should drop everything and devote every moment of your life to fitness and dieting! Starting small, even if just snacking on a carrot instead of a donut once in a while and taking a short walk can go a long way toward improving your long-term health.
Keep track of your blood pressure at least monthly (and more frequently as you get older).
Get enough quality sleep and keep a regular sleep schedule.
Try to reduce chronic emotional stress. We understand that it’s not an easy feat in modern times, but it’s been shown that high levels of stress can cause increased cortisol and affect your cardiovascular health.
Monitor cholesterol and blood sugar. These are some of the common lab tests that are prescribed to check for heart disease risks. Ideally, make sure that both LDL and HDL cholesterol are included, since they perform very different roles.

Don’t forget to bring your Heartify report when you go to your doctor’s appointment! You can save the logs of your heart rate, HRV, and blood pressure so that you’re prepared for the conversation. Some research supports “white coat hypertension” which means that some people might experience higher blood pressure and heart rate when measured in a clinical setting.

A Word on Blood Pressure

Wouldn’t it be great if you could measure your blood pressure anywhere without a special device? We think so, too! However, as of today there aren’t any proven ways to do that. There have been a lot of improvements and breakthroughs in recent years, and some tech manufacturers have created compact prototype devices that aim to make blood pressure recording easier, but there is not nearly enough evidence to support that they are accurate enough. The current consensus is that arm-cuff type blood pressure monitors are still superior to all others in terms of accuracy. For at-home monitoring you can use wrist monitors, but not all of them have shown high precision. But you can be sure that we’re staying on the forefront of technology and we’re always on the lookout for proven monitoring technologies.

References

Park, J., Seok, H. S., Kim, S., & Shin, H. (2022). Photoplethysmogram Analysis and Applications: An Integrative Review. Frontiers in Physiology, 12, 808451. https://doi.org/10.3389/fphys.2021.808451
Usui, H., & Nishida, Y. (2017). The very low-frequency band of heart rate variability represents the slow recovery component after a mental stress task. PLoS ONE, 12(8). https://doi.org/10.1371/journal.pone.0182611
Heathers, J. A. (2014). Everything Hertz: Methodological issues in short-term frequency-domain HRV. Frontiers in Physiology, 5, 57372. https://doi.org/10.3389/fphys.2014.00177
Shaffer, F., & Ginsberg, J. P. (2017). An Overview of Heart Rate Variability Metrics and Norms. Frontiers in Public Health, 5. https://doi.org/10.3389/fpubh.2017.00258
Lee, G., Song, Y. D., & Lee, E. C. (2023). Experimental Verification of the Possibility of Reducing Photoplethysmography Measurement Time for Stress Index Calculation. Sensors (Basel, Switzerland), 23(12). https://doi.org/10.3390/s23125511
Baevsky, R. M., & Chernikova, A. G. (2017). Heart rate variability analysis: Physiological foundations and main methods. Cardiometry, (10), 66-76. https://doi.org/10.12710/cardiometry.2017.10.6676
Kim, S., Yoon, H., & Cho, H. (2014). Diurnal Heart Rate Variability Fluctuations in Normal Volunteers. Journal of Diabetes Science and Technology, 8(2), 431-433. https://doi.org/10.1177/1932296813519013
Bigger, J. T., Jr, Fleiss, J. L., Steinman, R. C., Rolnitzky, L. M., Kleiger, R. E., & Rottman, J. N. (1992). Frequency domain measures of heart period variability and mortality after myocardial infarction. Circulation, 85(1), 164–171. https://doi.org/10.1161/01.cir.85.1.164
Shah, A. J., Lampert, R., Goldberg, J., Veledar, E., Bremner, J. D., & Vaccarino, V. (2013). Posttraumatic stress disorder and impaired autonomic modulation in male twins. Biological psychiatry, 73(11), 1103–1110. https://doi.org/10.1016/j.biopsych.2013.01.019
Carney, R. M., Freedland, K. E., Stein, P. K., Miller, G. E., Steinmeyer, B., Rich, M. W., & Duntley, S. P. (2007). Heart rate variability and markers of inflammation and coagulation in depressed patients with coronary heart disease. Journal of psychosomatic research, 62(4), 463–467. https://doi.org/10.1016/j.jpsychores.2006.12.004