How To Improve HRV With Better Sleep

What is Heart Rate Variability or HRV?

Many wearables can now show you your Heart Rate Variability (HRV) score. What exactly is HRV? Your heart rate is the number of heartbeats in one minute. HRV measures the fluctuation of time between heartbeats. When you are awake, a high rate of change between heartbeats (i.e. a high HRV) reflects a healthy heart. A healthy heart does not beat evenly like a metronome. Having a higher rate of variability from one heartbeat to the next indicates the ability to quickly adapt to changes in the environment.

Why should I care if my HRV is low?

Your heart is controlled by the part of your brain that does not require active thought to function, your autonomic nervous system. There are two branches of your autonomic nervous system. The first is active when you are stressed, sometimes referred to as your “fight or flight mode” and is represented with a higher heartbeat. The other branch is in control when you are relaxed, with a lower heartbeat, sometimes referred to as “rest and digest mode.” If the variability between heartbeats (your HRV) is low, it usually indicates an overall higher level of stress, meaning your heart rate is spending too much time in fight or flight mode. A low HRV indicates a need for mental and physical recovery from stress.

A low HRV can predict:

• poor sleep quality 
• depression
• increased anxiety
• heart problems
• a need for rest and recovery 
• overall poor health
• death at an earlier age

Poor sleep quality has separately been correlated with the other issues listed above and may be the main cause of low daytime HRV readings. In Figure 1, Poor sleep quality measured by a sleep questionnaire shows that a low HRV is a significant predictor of poor sleep quality. On the contrary, a high HRV does not convincingly predict good sleep quality. It is possible to have a relatively higher HRV and still have sleep issues. However, if your HRV is low, you almost certainly have sleep quality or quantity issues that need to be resolved.

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Figure 1: A low HRV is predictive of low sleep quality. The "p" values reflect the likelihood that the result is due to chance. In this case, a p-value of .001 means that there is only a 0.1% possibility that the above result is due to chance rather than a low HRV reading. Conversely, there is a 46.3% chance that the comparison group data is based on chance rather than their HRV score. In other words, a low HRV predicts low sleep quality, but a high HRV does not predict good sleep quality. Adapted from (Estrela 2021).

A long-term study seen in Figure 2 shows that having a chronically lower HRV will, on average, lead to an earlier death. Most of these deaths are related to heart malfunctions caused by poor sleep, a poor diet, drug or alcohol use, and a lack of exercise.

Figure 2: A lower-than-average HRV is associated with an earlier death mostly due to cardiac malfunctions. Adapted from (Hämmerle 2020).

How is HRV calculated?

Figure 3 shows how the raw data is compiled to determine HRV. The “RR interval” in Figure 3A is the measurement of time between heartbeats. The RR intervals in Figure 3B show the variation of RR over 2-3 hours, while Figure 3C shows all of the RR heartbeat data used to calculate HRV. Tracking devices such as rings and watches use the data shown in Figure 1C to calculate the RSMMD, the HRV number that is reported on your device. The RSMMD reflects the variability between heartbeats over 5-10 minutes. For those who want to know the exact calculation, RSSMD is the root mean square of consecutive differences between the RR intervals. Yes, that is too much information for us non-math majors! All you need to know is that the time of your RR intervals should be varied. If the amount of time between your heartbeats (RR) is similar, your HRV will be low. If the time between heartbeats is diverse, your HRV will be high. High variations of time between heartbeats reflect a healthy nervous system ready and willing to respond appropriately to environmental stressors.

Figure 3: RR intervals in "A" show the time between heartbeats on an electrocardiogram. In "B" variations in heart rate intervals (R-R) are seen over 2-3 hours on a tachogram. "C" reflects all the R-R data used to calculate an “RSSMD” HRV score. Adapted from (Hammerle 2020).

How do HRV measurements differ during sleep compared to being awake?

Your HRV will be lower while asleep than when you are awake. Since you are in a relaxed state while sleeping, you should have fewer stress responses than when you are active during the day. In general, your average nighttime HRV score should be 20-30% lower compared to your daytime score. If it is not at least 20% lower, this can be an indicator of poor sleep quality which needs to be addressed. 

HRV does change depending on which part of the sleep cycle you are in. it will be higher when your brain is actively dreaming during REM sleep than when you are in deep or light sleep stages (non-REM sleep). In Figure 4, patients who recently had heart attacks had a higher HRV while sleeping than they do while awake! The healthy heart controls in Figure 4 have higher HRV readings during REM sleep than non-REM sleep, but their HRV scores are still 20% or more lower than when they are awake. If your HRV scores are higher while sleeping than while awake, please make an appointment with a cardiologist as soon as possible!

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Figure 4: Post-MI represents post-myocardial infarction (i.e. post heart attack) patients. Instead of the RSSMD seen on most wearables, a ratio of low frequency to high frequency R-R values were graphed. Healthy LF/HF ratios are similar to RSSMD scores where a higher number is an indication of better health while awake. Healthy REM sleep values for LF/HF should be an average of 20-30% lower than scores measured while awake.

Factors affecting HRV scores.

• Poor sleep and recovery
• Age – HRV is highest in young adulthood and decreases with age.
• Sex – women have a lower HRV compared to men (spend more time in the rest and digest phase) until menopause.
• Pregnancy – pregnancy causes a decrease in HRV.
• Circadian rhythms – measure your HRV at the same time every day. HRV goes up as you are awake, and down as you sleep.
• Ethnicity – HRV is higher in African Americans compared to those of European origin.
• Metabolic disease reduces HRV.
• Cardiac disease reduces HRV.
• A large number of psychiatric disorders correlate with a reduced HRV.
• Various other diseases can cause a low HRV.
• Alcohol and drug consumption lower HRV.

How can I increase my HRV?

‍The answer to this question is all about reducing stress. 

• Increased sleep quality and a duration of 7-9 hours
• Exercise
• Weight loss via healthy eating 
• Meditation
• Breathing exercises
• Stop drug use and smoking
• Reduce or eliminate alcohol intake
• Reduce or eliminate caffeine intake
• A low HRV is associated with increased aging. HRV is highly correlated with heart health. A low HRV is a marker used to predict heart attacks, high blood pressure, sudden cardiac death, and an irregular or inefficient heartbeat. One of the ways you can increase sleep quality along with your HRV is to exercise. Figure 5 shows HRV (RSSMD scores) from older, masters athletes (athletes aged over 40) compared to non-exercising controls. There was no significant difference between the older athletes and the young controls. However, there was a huge difference between non-exercising older adults and exercising adults.

Figure 5: Masters athletes are athletes performing at the top 25% of their age-group for 15 years or more in either running or triathlon. RSSMD numbers are used for HRV scores. Adapted from (de Deus 2018).

Athletes tend to have a higher sleep quality than non-athletes in controlled environments. Regular exercise can help you sleep better if it is completed no more than two hours before bedtime. Getting quality sleep is the best way to increase HRV, but sleep disorders may prevent you from getting good sleep despite exercise. Solving your sleep issues is paramount to increasing your longevity and overall health!

References

Vanoli, E., Adamson, P. B., Ba-Lin, Pinna, G. D., Lazzara, R., & Orr, W. C. (1995, April). Heart Rate Variability During Sleep Specific Stages. Circulation, 91(7), 1918-1922.

Alves de Deus, L., Sousa, C. V., Rosa, T. S., Filho, J. M., dos Santos, P. A., Barbosa, L. D., . . . Gustavo, H. S. (2019, June 1). Heart rate variability in middle-aged sprint and endurance athletes. Physiology and Behavior, 205, 39-43.

da Estrela, C., McGrath, J., Booij, L., & Gouin, J.-P. (2021, April). Heart Rate Variability, Sleep Quaility, and Depression in the Context of Chronic Stress. International journal of psychophysiology : official journal of the International Organization of Psychophysiology, 162, 79-85.

Hämmerle, P., Eick, C., Blum, S., Schlageter, V., Bauer, A., Rizas, K., . . . Swiss-AF Study Investigators. (2020, July 28). Heart Rate Variability Triangular Index as a Predictor of Cardiovascular Mortality in Patients With Atrial Fibrillation. Journal of the American Heart Association, 9(15).

Sammito, S., Thielmann, B., & Böckelmann, I. (2024, August 5). Update: factors influencing heart rate variability–a narrative review. Frontiers in Physiology, 15.

Shaffer, F., & Ginsberg, J. (2017, September 28). An Overview of Heart Rate Variability Metrics and Norms. Frontiers in Public Health, 5(258).

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