There’s a surprisingly simple but little-known technique for measuring your in-the-moment ability to handle physical, mental and emotional stress. Some of us have a higher level of “adaptive reserves” than others. And for all of us, these adaptive reserves increase or decrease throughout the day and over days or weeks. You might have a rough, intuitive sense of when you are more resilient to stress, and when you are weak or more vulnerable. There is no shortage of advice on the Internet about “stress management”, fitness and diet. In general, sleep, physical activity and a nutritious diet build adaptive reserves, while these reserves are easily depleted by chronic stressors like worry or overwork.
But how do you know what really works to improve your resilience and resistance to physical and psychic stressors? How can you learn what diet, exercise, and activities actually work for YOU — not just what researchers, health professionals, bloggers or your friends claim should work?
What if I told you that there is a reasonably objective metric for your overall adaptive reserves and a simple, inexpensive, and noninvasive way to measure your adaptive reserves throughout the day.
The metric is called Heart Rate variability, or HRV. It can be measured in less than a minute, using an inexpensive chest strap or finger sensor, and any of several smart phone apps that can be downloaded for a few dollars. Based on a review of numerous studies and extensive personal experimentation, I’ve learned some surprising things about what tends to raise or lower HRV. And I’ve found one few amazingly quick, effective, and to boost a low HRV and keep it up, with real impact on adaptive reserves. (For the impatient, scroll to bottom of this post and read Recommendation #1).
Athletes and the rest of us. Endurance athletes and weightlifters have been using HRV measurements for years to improve performance, while avoiding overtraining and injury. The technique was first piloted using sophisticated electrocardiogram equipment in exercise physiology labs. With the advent of inexpensive Heart rate monitors and smart phone software, the HRV technique is available to amateur athletes.
But the exciting thing is that Heart Rate Variability has recently found applications well beyond optimizing athletic performance. Low or altered HRV has been found to predict progression of diseases as diverse as cardiovascular disease, cancer, autoimmune disorders, infections, trauma, post-traumatic stress disorder, and depression.
Heart rate variability turns out to be a generalized, deep measure of health. That’s because higher HRV is a strong indicator of resilience to stress, while low HRV is a sign of reduced capacity to tolerate stress. And at the deepest level, health is resilience, and diseases in various ways compromise resilience.
What is HRV anyway? It is well known that a lower heart rate or pulse is associated with fitness. At rest, an average adult’s heart beats between about 60 and 90 beats per minute, whereas fit athletes can have resting heart rates as low as 45 bpm. As a general rule, a lower heart rate is a good thing, because it indicates a more powerful, efficient heart. However, a low heart rate can also reflect genetic differences, or medicals conditions like brachyardia.
But heart rate is not the full story. Healthy hearts don’t beat like a metronome. The intervals between each beat vary. If you have a resting heart rate of 60 bpm, your heart beats on average once per second. Yet if you precisely measure your heart beat, the actual beat-to-beat intervals (often called NN intervals) may be something like:
1.05 sec….0.91 sec….1.01 sec….1.07 sec…..0.93 sec….1.02 sec…1.03 sec….0.98 sec….
The average here is 1.00 seconds, but each interval varies somewhat from the average. That’s heart rate variability.
HRV and the autonomic nervous system. The primary source of variability in your heart rate is the autonomic nervous system (ANS). The pacemaker in your heart, the sinoatrial (SA) node, receives input from the two halves of your ANS — the sympathetic nervous system (SNS) and the parasympathetic nervous system (PSNS). You may recall from your biology class that the sympathetic system arouses us for vigilance and action — so called “fight or flight” activity, whereas the parasympathetic system puts the brakes on, calming us down after action and promoting relaxation, digestion, sex and other activities best saved for times we are not faced with immediate danger or urgent priorities. The parasympathetic system also supports the prefrontal cortex in rational activities like planning and decision-making. It strengthens willpower!
Sympathetic and parasympathetic systems “compete” with each other for control of the heart. The sympathetic system is activated by the accelerans nerve, which releases norephinephrine (“adrenaline”) into the to the heart’s SA node, stimulating it to beat faster and more steadily. The parasympathetic system works via the the vagus nerve and the release of acetylcholine, slowing the heart rate and introducing more variability, especially in the higher frequency (HF) ranges.
In short, a dominant sympathetic system leads to reduced heart rate variability, while a strong parasympathetic system increases HRV.
A healthy person has strong sympathetic and parasympathetic systems and a good balance or ability to shift between the two. A weak sympathetic system leaves you slow to arouse to action and lethargic, and weak parasympathetic system can leave you hyperactive, overly reactive, and slow to calm down.
Stress and the sympathetic nervous system. While both of these are real problems, I think its fair to say that the more significant problem in our industrial society is an overactive sympathetic system and underactive parasympathetic system. A weak parasympathetic system leaves you unable to dial down arousal and overly reactive to physical and emotional challenge. If the sympathetic system is dominant and not adequately checked by the parasympathetic system, your physical and emotional batteries are never “recharged”, leaving them depleted. An overactive sympathetic nervous system that never turns down can also lead to numerous physical and psychological health problems as mentioned above.
This balance or “dance” between the two halves of the ANS is reflected not just in heart rate variability, but also in hormonal balance. Stress hormones — most prominently cortisol, but also norephinephrine and ephiephrine — rouse us to action. Cortisol is associated with sympathetic activation — typically highest upon waking in the morning, tapering off as the day wanes. That’s helpful for energizing us to action. But chronic and unrelieved stress, either from physical fatigue or an overload of mental, emotional and social stressors, result chronically elevated cortisol levels. This leaves us anxious and burnt out, and can lead to sleep problems and depression if not corrected.
While an imbalanced ANS can be detected by running hormone tests, that’s inconvenient and expensive and there is (at least so far) no instant cortisol test that provides real time feedback.
That’s where HRV measurement comes in. It’s cheap, convenient and fast. You can measure your HRV in less than a minute, any time of day, at no cost beyond the initial outlay of $40 – $100 for a chest strap heart monitor and a smart phone app.
Why is “variability” good? Another name for heart rate variability is “arrhythmia”. But isn’t it bad to have a heart arrhythmia? It turns out that there is “normal” arrhythmia and pathological arrhythmia, and we usually hear only about the latter. But in general, a variable heart rate is a sign of health. High HRV is a sign of the flexibility of the heart and the capacity of the autonomic nervous system to adapt to changes in the demands we face every day. It’s fundamental to the idea of homeostasis — the ability of an organism to remain dynamically centered in the face of an unpredictable environment. This is something that physiologists have come to realize over the past 30 years. James Gleick eloquently explained why variability is good in his classic book “Chaos” :
Now some physiologists speak of dynamical diseases: disorders of systems, breakdowns in coordination or control. “Systems that normally oscillate, stop oscillating, or begin to oscillate in a new and unexpected fashion, and systems that normally do nor oscillate, begin oscillating,” was one formulation. These syndromes include breathing disorders: panting, sighing, Cheyne-Stokes respiration, and infant apnea – linked to sudden infant death syndrome
….But physiologists have also begun to see chaos as health. It has long been understood that nonlinearity in feedback processes serves to regulate and control. Simply put, a linear process, given a slight nudge, tends to remain slightly off track. A nonlinear process, given the same nudge, tends to return to its starting point. (Chaos, p. 292)
Healthy variability shows up not just in the heart rate, but in a wide range of physiological features — gait, speech, and even thinking. You can see the loss of variability in the way that victims of strokes or Parkinson’s disease walk and talk with rigidity. By contrast, children and healthy adults have variable physiologies that make them resilient when faced with the unexpected.
How is HRV measured? There are a number of different ways that physiologists use to characterize this variability. The simplest is the standard deviation. For the example above, the standard deviation of the NN beat-to-beat interval (SDNN) is 0.054 seconds, or 5.4% of the mean. There are many other ways to characterize HRV. Some, like SDNN are “time-domain” methods because they look at the variability in seconds. Others, called “frequency-domain” methods look at the percentage of intervals that fall within certain frequency ranges. The research on HRV gets very sophisticated, but it seems that one of the most consistent metrics for parasympathetic strength is RMSSD or “root mean squared difference” between adjacent N-N intervals. Many researchers and athletes mean RMSSD when they speak of HRV.
Heart rate can be measured using a heart rate monitor that straps around your chest, or with a finger sensor, or just by measuring your pulse with your fingers. But heart rate variability requires precise measurement and software to capture the variations and reduce them to a number. So you’ll need a chest strap heart monitor and an app for your iPhone or Android phone from a company like Bioforce, Sweetbeat, Elite, Inner Balance or Ithlete. I chose Ithlete based upon reviews indicating it provides consistent, dependable results, and it was also compatible with my older iPhone 4 by using an insertable receiver. (Newer phones can receive signals directly via Bluetooth). I’ve been happy with the Ithlete.
One funky thing about Ithlete and Bioforce is that they do not report HRV using the common RMSSD measurement, but instead use lnRMSSDx20. That just means 20 times the natural logarithm of RMSSD. They chose this alternative metric in order to spread out the values and represent HRV using a more “natural” scale with 100 being something like perfect fitness. (Highly fit individuals could actually reach higher than 100 on this scale, but rarely).
What is a “good” HRV? There’s no universal answer to this, but we can take some clues from a 2010 American Journal Cardiology study by Zulfigar et al. at the U. Illinois Medical Center. They measured the HRV of 344 healthy subjects ages 10-99. Keep in mind that these HRV values were calculated using data over a full 24-hours. As excerpted by Simon Wegerif on the Ithlete Blog, here is a plot from the Zulfigar study, showing the distribution of RMSSD values by age. It shows a lot of variation by age, but a clear trend towards lower HRV as one gets older:
Based upon this graphical plot, we can put together a table that gives some guidance as to what is a good HRV for people of different ages. I’ve tabulated the data using both RMSSD and the lnRMSSDx20 measure used in the popular Ithlete and BioForce apps.
Again, keep in mind that these HRV values represent 24-hour averages. If you are measuring your own HRV, you’ll find that it tends to be a little lower in the morning than later in the day, and it will vary based upon physical activity, diet and other factors. While I frequently get HRV readings above 80 (I’ll be henceforth using the lnRMSSDx20 scale) which is “superior”, I often get very low HRV readings, particularly right after a vigorous workout.
Overall, my personal average HRV taken multiple times each day over a month is 68 – a fair bit better than average (62), but not “athletic” (72-79) for someone close to 60 years old. A common practice for looking at long term trends is to measure your HRV first thing in the morning, before food or activity. If I do that, my average HRV is slightly lower — about 66. But it changes a lot depending on my behavior. Some days, my waking HRV gets into the 70-80 range, and it has dropped below 50 on one occasion.
Heart rate as a proxy for HRV? Heart rate variability tends to increase as heart rate decreases, and vice versa. So you might be thinking: Do I really need to buy a heart rate monitor to measure HRV? Can’t I just take my pulse as an “inverse proxy” for HRV? The chart below shows the data I collected on myself over a months. It taught me that a lower heart rate of around 60 or less was associated with very good HRVs in the range of 70-90. Unfortunately, the correlation is very approximate. Using my own data, it is clear that if I were to just use heart rate, my estimate of HRV could be off by 20 points! If I measure a heart rate of 80, my HRV could be as low as 53 (poor) or as high as 78 (excellent). It’s also important to point out that this relationship between heart rate and HRV varies a lot between individuals. Some athletes with very low heart rates can have poor HRVs if they are overtrained. So if you want to do this the right way, buy the chest strap and download the app!
By tracking my HRV like a nerdy Quantified Self enthusiast, I’ve learned a lot about how to boost HRV. And I’m very excited to learn that there are really only a few things that make a big difference.
A Short Quiz: What affects HRV? Take a few minutes to take this quiz before reading on. I’ll provide the answers further down, but it’s good to test your intuition first.
What follows is a numbered list of activities. Write down the numbers 1-10 on a piece of paper. For each number write “increase” or “decrease” based on the effect you think the action will have on HRV.
- Running on the treadmill for 30 minutes
- Lifting heavy weights for 30 minutes
- Eating a high carb meal (pasta, bread, ice cream)
- Eating a high fat, low carb, moderate protein meal (avocado and shrimp salad)
- Fasting for 12-20 hours
- Fasting for 48 hours
- Drinking a beer or glass of wine
- Drinking 3 beers or 3 glasses of wine
- Taking a cold shower for 2 minutes
- Soaking in a hot tub for 15-30 minutes
Here come the answers:
I’ll emphasize that these answers are based on a combination of my personal experience and the research literatures. Since we are all wired a little differently, there may be variation in how you would respond. And my limited research in this emerging field should be taken as preliminary, and not the final word. Nevertheless, here’s what I learned:
Exercise. (Items 1 and 2).
Running and weight lifting, and virtually any exercise, raises heart rate and lowers HRV in the short term. Sometimes quite significantly. I found that both running and indoor rock climbing significantly reduced my HRV – by as much as 40-50 points initially! My HRV values gradually climbed back up over several hours. The more intense the exercise, the slower the recovery. A short walk lowered HRV only briefly.
Essentially the same effect of intensity on slower HRV recovery has been documented in an Australian study reviewed by Simon Wegerif on the Ithlete blog.
So why exercise if it reduces your resilience? The answer is that the effect is quite temporary. Exercise boosts your HRV higher over time, as you become more fit. This is a classic example of hormesis: initial deterioration leading to longer term super-compensation. The trick is to moderate the intensity and frequency of exercise so that the long term gains exceeds the transient losses.
Diet and fasting. (Items 3-6).
I did not personally find any negative impact of the carbohydrate or fat composition of my food on my HRV. However, I may not be the ideal subject, because my consumption of carbohydrates is fairly modest and intermittent. A 2010 study comparing high and low carbohydrate diets found the HRV values from both diets to be similar.
Some have claimed that food sensitivities cause heart rate to increase and HRV to decrease. Dave Asprey has worked with Sweetwater to develop an a free “Bulletproof Food Sense” app that claims to identify your “hidden” food sensitivities. He also has developed a “HeartMath Inner Balance” app that trains meditation patterns designed to increase HRV. I have not really researched science behind these ideas or tried then myself, so I can’t vouch for these products. But it may be worth looking into for those who are interested.
I have personally found that intermittent fasting significantly increased my HRV. On days I that I fasted through the morning and afternoon, my average HRV was 78 — much higher than my average of 68 — and much of the time it was above 80! I also found that the longer I went without eating, the higher my HRV climbed, although I have not fasted more than 20 hours at a stretch this past month.
A 2014 study in Aging Cell found that individuals who consume fewer calories have significantly higher HRV, across the age spectrum. It also appears that regular short term fasting for less than a day tends to raise HRV, but fasting for 48 hours reduces HRV. A 2014 study of daily fasting during Ramadan, followed 16 women and 24 men during the a month of fasting about 17 hours per day between sunrise and sunset. The study found a statistically significant increase in HRV with high frequency gains indicative of parasympathetic strengthening. RMSSD increased from 31.1 to 36.3 (from 68.7 to 72.2 in lnRMSSDx20 units).
However, longer term fasting may not be beneficial, at least without a period of adaptation. A 2013 study in the European Journal of Clinical Nutrition found a small improvement in RMSSD at 24 hours. However, they also found a significant decrease in HRV at 48 hours, indicative of “parasympathetic withdrawal with simultaneous sympathetic activation”, which the authors suggest are reflective of stress.
Alcohol.(Items 7 and 8).
This one is a bit of a bummer. Not too surprisingly, alcohol suppresses HRV. It makes sense that alcohol impairs your stress tolerance. The good news is that a single beer or glass of wine dropped my HRV by no more than 5 or 10 points, sometimes not having any effect. However, 2 or 3 beers or glasses or wine, or a few cocktails consumed within a few hours — while quite pleasant — really sent my HRV into a downward spiral, sometimes by as much as 35 or 40 points, to a low of 36.7 points! Ouch! Interestingly, the depressive effect on HRV typically took several hours to show up and continued well after I had stopped drinking and supposedly “cleared” the alcohol.
This is buttressed by the research. A 2010 study in the American Journal of Physiology and two 2013 studies in Psychophysiology and Alcohol Clinical Experimental Research found that moderate alcohol consumption (1 drink per day for women, 2 drinks for men) increases HRV, while heavy alcohol consumption reduces it.
This really makes a case for moderate alcohol consumption!
On the flip side, there is some evidence that caffeine increases HRV, though the effect is transient (20-40 minutes) and may be dose-dependent.
Hot and cold. (Items 8 and 9).
I’ve saved the best for last. The activity that has delivered the single biggest improvement to HRV for me has been…cold showers! This has also been the most consistent and most immediate of all the measures I’ve tried. With over 40 cold showers tested — both in the morning and other times of day, cold showers increased HRV by an average of 8.4 points. In many cases, particularly when HRV started low, the increase was 20 points or higher!
Conversely, heat speeds up the heart and reduces HRV. Soaking in the hot tub dropped my HRV by an average of 9.7 points, and the longer I stayed in, the lower it dropped.
I find this particularly remarkable for several reasons:
- I always assumed that cold showers increased my heart rate, which usually means a decrease in HRV. (See again my plot above). In my original post on Cold Showers, I wrote: “When you start with cold water, you will experience the phenomenon of cold shock, an involuntary response characterized by a sudden rapid breathing and increased heart rate.” But it appears that just the opposite is true — the heart slows down in the cold. Apparently this is well-known! I does make sense physiologically that the body conserves heat by pumping more slowly, thereby keeping internal organs warm and reducing heat loss through the skin.
- It was also surprising to me that the hot tub lowered my HRV and sped up my heart. Because the hot tub is so relaxing, I assumed everything is slowing down. I was wrong. But it makes sense that as you warm up, the heart pumps faster in an attempt to dump heat through your skin. That may not be particularly effective if the water is very hot, of course.
- The HRV-raising effect of cold shower is not transient. It really does a “re-set” and bumps your HRV up for hours. If you take a cold shower before bed, you wake up with a similar HRV in the morning.
- Apparently the HRV-raising effect of cold exposure–which I was ignorant about before self-experimentation– is well known to physiologists. Strengthening the parasympathetic branch of the ANS is enhanced only after some acclimatization. (See here and here for references). So while cold showers universally boosted my HRV, it may take you several attempts to see the benefit if you are unadapted to cold water.
- If you find it hard to take cold showers, or you like “contrast baths” that involve switching between hot springs and cold water; these findings give a good reason to end your shower or bath with cold water, rather than hot.
- Cold showers are quite effective in reversing, almost erasing the HRV-lowering effects of exercise and alcohol. In fact, on nights where I’ve had a few drinks and my HRV is low, I make sure to jump into the cold right before going to bed. Perhaps this sounds awful. But it really makes you feel great before hitting the sack! And because it re-sets your HRV, you wake up with a higher HRV in the morning than if you went to bed without it. That’s incredible – lowering your heartbeat all night long! If it’s true that we all have a finite number of heartbeats in life, why not spread the heat beats out in time, particularly when you are asleep, and perhaps add a few years to your lifespan or healthspan!
I’m particularly excited because this cold shower effect on HRV provides a great explanation for another observation I made in my Cold Showers post:
“…cold showers appear to have improved my stress tolerance, by buffering emotional reactions. What I mean by this is that bad news, surprises, arguments, or events that would have previously caused a brief surge in adrenaline or an emotional flush, no longer have that effect, or at most have a very attenuated effect.”
There you have it: Cold showers boost HRV and activate the parasympathetic nervous system, calming reactivity and strengthening our adaptive reserves!
Recommendations. Pulling this all together, the research and my personal experience suggest that tracking your heart rate variability with a chest monitor and smart phone can help you gain a lot of insight into best to adjust your diet, exercise program, and lifestyle in order to improve your ability to handle stress.
Based upon what I’ve learned from the literature and self-experimentation, I can suggest four simple and powerful steps to boosting your HRV and becoming more resilient:
- Jump into a cold shower or take a cold swim for a few minutes in the morning to arm yourself against the stresses of the day. It only takes a minute or two under the cold water to raise your HRV significantly! Use cold showers after intense exercise or drinking alcohol to more quickly get back on track before you go to bed, so you wake up fresh each day!
- Exercise judiciously. The harder you train, the more time you need to recover between sessions in order to avoid overtraining.
- Use intermittent fasting part of each day to power up your parasympathetic nervous system. But be careful fasting more than a day.
- Use alcohol and caffeine moderately. An alcoholic drink or two a few times a week can increase your resilience, but more than a few drinks can really set you back. Likewise, a cup or two of coffee can be helpful, but be wary of over-caffeinated energy drinks.
In all of the above, a personal heart rate monitor can help you decide whether the advice works for you, and can help you fine tune the extent and frequency of each practice that works best for you.
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This post first appeared on Train Yourself To Thrive On Stress / Getting Stronger, please read the originial post: here