What can you learn from tracking your HRV while sleeping?

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Heart Rate Variability (HRV) gained tremendous attention in recent years as an indicator of mental and physical health.

HRV corresponds to the time difference between two consecutive heartbeats. Our heart doesn’t beat like a metronome and the slight shifts, measured in milliseconds between two heartbeats, are under the control of the autonomic nervous system.

Analyzing the HRV is looking at how well our mind and body cope with stress. Modern Wearables and the powerful algorithms and sensors that sustain them offer a convenient way to analyze our health, mental status, and overall behavioral flexibility.

Heart Rate Variability is an independent mortality risk factor that can be of particular interest when monitored during sleep. This article will review the latest discoveries and scientific studies that could help understand the predictive value of heart rate variability during sleep.

In 2004, a team led by Julian Thayer analyzed the effect of pre-sleep acute stress on the Heart Rate Variability decrease and had a prognostic factor for increased risk of mortality.

It is well established that type II diabetes is one of the significant contributors to cardiovascular events. In addition, a decrease in sleep HVR recorded in diabetic patients proved an essential predictor of cardiac incidents. 

Relationships between HRV and the Autonomic Nervous System

The Heart Rate Variability is an easy and efficient way to identify imbalances of the Autonomic Nervous System. The HRV can help assess the Autonomic Nervous System (ANS) flexibility capabilities.

The more stressed you will be, the less resilient your autonomous system and the lower the HRV

It all comes to the two branches of the ANS: sympathetic and parasympathetic, respectively responsible for preparing the body to fight-or-flight or relaxing. One will increase the heart rate, and the other one will reduce the heart rhythm.

Why would you measure HRV at night?

We, mammals, spend at least one-third of our lives sleeping, which emphasizes the critical need for sleep at physiological and psychological levels.

In a study published in 2020 including 260 participants, the team of Pr. Kelishadi found that poor sleep quality was adversely correlated with Heart rhythm, HRV, and Blood Pressure.

Sleep cycles also proved to impact the HRV, with Heart Rate Variability decreasing during the REM sleep (Rapid Eye movements or dream) phase. Even more, the intensity of the dreams appeared to impact the RR intervals (the time between two consecutive peaks on an electrocardiogram.)

Thus more than opening a window on the sympathetic activity, and physical or cardiac health, measuring the HRV could also help detect the various sleep cycles more precisely

Obstructive sleep apneas and their impact on HRV

Obstructive sleep apnea is also associated with a decrease in Heart Rate Variability. The type of respiratory event and impact on the reduction of RR duration also proved informative in defining the potential cardiovascular underlying risks

Nocturnal HRV and sudden death risks after Myocardial Infarction

Interestingly after Myocardial Infarction, vagal activation is lost, resulting in the loss of nocturnal HRV. This decrease has been suggested as a predictor for the occurrence of sudden death while asleep in people suffering from post-myocardial infarction. 

Sleep Quality and HRV to assess the development of the ANS in children

It is well known that insufficient sleep, especially in children, will carry consequences and impair children’s physical and psychological development. 

In 2013, a team at the Faculty of Medicine and Health Science in Ghent, Belgium, investigated the effect of low sleep quality in a population of children not reported as suffering from low sleep duration, as reported by their parents.

The team assessed the quality of sleep using polysomnographic studies and assessed its impact on Heart rate Variability. When adjusted for stress, a significant decrease in HRV was found. 

This study was the first one to focus on the impact of sleep quality on HRV in Children. Sleep quality, not only sleep duration, is a critical factor for the healthy development of the autonomic system occurring during childhood.

How to collect HRV data at night?

Now that we have established the importance of measuring the sympathetic activity at night and the cardiovascular disease predictive value of HRV, we may wonder how to collect the relevant data.

The studies we described so far were primarily conducted in sleep laboratories equipped with the latest polysomnography and heart rate monitoring. Unfortunately, we do not all have access to such accurate technologies, and we mostly rely on smartwatches or other types of consumer wearables.

Most wearable devices measuring HRV will either rely on electrodes or photoplethysmography to record heart rhythm and extract RR intervals.

Few devices are capable of continuously measuring the HRV. Some, including the Apple Watch, will provide spot measurements, while others, such as the YHE BP Doctor, will only measure the HRV during rest or night.

HRV is a robust biomarker of the Autonomic Nervous System’s ability to adapt. However, it is most informative when used over a long period to allow the extraction of trends.

According to the Guidelines put forward by the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, it is recommended to measure the HRV either long term (24h) or short term (5 min).

Continuous 24 hours HRV measurement is optimal for analyzing the cardiovascular system response to various stimuli and situations across the circadian rhythm. 

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What are the best devices for continuous HRV recordings?

Dr. Hide from the Human and Social Science Group of the Defence and Science Technology Laboratory in the United Kingdom recently analyzed what devices would be more suitable for monitoring twenty-four hours Heart Rate Variability in military populations.

Even though the end goal was to detect devices suitable for military personnel, the results of the thirty-two devices compared are especially interesting as it allows to potentially identify robust and efficient devices that could be used by non-military.

Eights factors were taken into consideration for identifying the best ones:

  • Is the device capable of continuous HRV measurement?
  • Does it measure a range of HRV parameters?
  • Does the device produce valid HRV measures?
  • Does the device have a battery life >24 hours?
  • Does the device have internal memory capacity?
  • Can the raw data be accessed?
  • Is the device robust?
  • Is the device suitable for military personnel?

Despite not being adapted for militaries, the Bittium FarosTM, Bodyguard 2, and Actiheart were the most reliable. Overall, even with some limitations in the measurement algorithms and its need to be paired with a smartphone or watch, the Polar H10 (Check on Amazon) was the best choice as long as accuracy and portability were concerned. 

To wrap up

The Heart Rate Variability is a convenient and efficient way to peak at the sympathetic Nervous System. Furthermore, reduced HRV is informative to assess physiological recovery and the risks of cardiovascular diseases.

Furthermore, HRV can also assess what sleep stage sleepers are currently in and differentiate rem sleep vs. non-rem sleep episodes. 

Sleep is an essential component of health, and sleep deprivation or sleep quality in adults or children are also visible in HR variability.

With the multitude of smartwatches and wearables currently available, choosing the right one to record heart rate variability during sleep can be complex, even though the Polar H10 offers the ability for accurate 24 hours continuous analyses.

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