Why you need to start tracking stress
Stress is many things. It can be annoying, or life destroying. It can be psychological, or physiological. We've covered definitions of stress before, so go read up if needed. The point is, whether you're an athlete looking to recover between workouts or a working professional looking to perform at your best, you can only control what you measure. Business 101. So if you want to avoid burnout and injury, you'll want a way to measure stress.
Good news, it's a thing. Unbeknownst to you all mere mortals, top athletes and CEO's have been using expensive tools to quantify and optimize their stress for years. I'm about to lay it on you, for free.
What is HRV?
In terms of clinical validation and mechanistic explanations of stress, the number one measure for stress across the board is heart rate variability (“HRV”). HRV as a number actually measures the time interval between heart beats, which is small but meaningful. The variability of that time interval beats is controlled by your central nervous system ("CNS").
HRV is therefore a measure of your nervous system balance, which is the trigger for stress management through your body. Low HRV means high sympathetic bias, i.e. fight or flight, a state of stress. Useful to evade predators, less useful to relax after a day at the office. High HRV means high parasympathetic bias, i.e. rest and digest. Dangerous in the presence of saber tooth tigers, but useful pretty much most of the time.
Why does HRV even matter?
The fact is that HRV is a downstream measure for stress responses your body triggers. It all goes through the CNS. So with one number, we can can capture the state of those stress responses. Pretty nifty, right?
If it's not already apparent, the reasons you might want to manage your stress levels include:
Chronic stress - Short-term acute stress can be good to keep you energized and alert to perform, but if you're unable to turn it off, you get into chronic stress territory. All the bad things happen here. Anxiety, depression, burnout, inflammation, even weakened immune response. Basically, you get sick and sad.
Recovery - So if you'd like to avoid all that bad stuff, then the way to titrate appropriate levels of stress is to measure recovery. That shows up as a drop in your HRV, or at least return to baseline day-to-day. A steady buildup of lower HRV means you haven't recovered. Less is worse, remember.
Longevity - If you're one of those guys that thinks you can sleep when you die, then your wish will come true sooner than you may think. To maintain your health well into advanced age, you'll want to now have chronic stress be a major storyline of your life. It burns the candle faster.
So if this one magic number encapsulates physiological and even psychological stress, how do we measure it, exactly?
How you measure HRV?
In recent years, as many professional athletes have adopted the technology with good results, we've started to see it trickle down into consumer grade devices like the Apple Watch and Oura Ring. We are yet to see HRV available in lower end devices around the $100 price point, though. Probably because even though you can use the same sensor as you do for regular heart rate, HRV requires a much higher sensitivity and lots of software to get a clean signal. So unless you already have that swag, what can you do?
Photoplethysmography, of course! Say that fives times, really fast. Okay, we can just use PPG for short. As such a nerdy scientific name implies, it was developed in clinical settings to use a photosensitive cell to detect changes in blood volume. Especially handy for applications where connecting leads is a challenge, as you don't actually have to touch the sensor. You point a bright light onto the skin, and measure how much light is reflected. The fun thing is that the amount of reflected light changes. Why? Because the volume of blood under the skin changes with the beating of your heart. So if you plot that changing signal on paper, it looks like a heart-beat. Because it is! Actually, it's also how those wearable devices do it. That's that tiny greed laser you see sometimes.
The thing is, the same thing can be done using your phone camera and your fingertip! Who knew you could do science with your smartphone? If you put your fingertip on the camera lens and activate the flash, the capillary veins in your fingertip are illuminated. You can't see any of that with the naked eye. It just looks like E.T.'s finger. But when you read the frames into software, there is a subtle pulsating effect that reflects your heart beat! All you need to do is tons of manipulation to the signal to remove noise and artifacts, and voila, it's like watching an ECG!
What's a good value for HRV?
Few things to establish. First, HRV isn't actually a unit of measurement. It's an overall representation of different ways to analyze those time intervals between beats. In fact, there are a whole host of measures for HRV, derived from time and frequency domain analyses. The main one you need to know about is the standard deviation of those time intervals, also known as SDNN. Some apps do use other numbers like RMSSD of LF/HF, so those aren't comparable with SDNN at all. Apple made a decision to use SDNN on the Apple Watch and Healthkit, so that's probably the winner here.
Perhaps even more important than what number you have access to is WHEN to measure. Like resting heart-rate, the sensitive nature of HRV makes it hard to isolate what exactly is affecting the result. Maybe you had a heavy lunch. Maybe you had a bad meeting. Maybe you just ran up some stairs. Or had coffee, or whatever. So measure it when you wake up, before you get out of bed! That is the most clinically valid measure, and allows you to then look at trends.
The trend is really the main event here, not the specific number you get. So let's break that down.
So what's normal?
Even harder to gauge than RHR, because to be frank there isn't as much research for HRV yet. Devices like Apple Watch that measure HRV have only been in the market a few years. What we do know about HRV (sdnn calculation) is that if you're close to 0, that isn't good. It means your heart pumps like a metronome, which isn't normal. If you're closer to 100, that's good. Between the two, it's harder to judge.
What do I do with this information?
Well, since it's far more sensitive than resting heart rate, the same advice applies. It can be a very accurate measure of what state your body is in. The trend is more important than the absolute number. If much lower than usual, say this week on average, then take it easy. Maybe go for a walk rather than a hard run. If high, then go out and conquer the world.
How do you get more?
So higher the better, pretty much. High-level answer: exercise. There is research (here and here) that shows some temporary changes through stretching, and some results through lifestyle interventions incl. diet and meditation, and some anecdotal evidence that shows different stressors like cold showers and sauna can boost your HRV. I’m sure we’ll see a lot more research on HRV in the future.
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HRV norms: PLOS