One of the most exciting features to arrive on fitness trackers in the past few years has been the daily rating of your stress levels out of 100. Admittedly for a while we found it mystifying, being unsure how to interpret or act on the information. That was until we chatted to an expert at Firstbeat, who explained that the rating is in part a measure of your heart rate variability using the optical heart rate monitor on the tracker.
Firstbeat is the company behind the algorithms that brands like Garmin use to rate your stress levels, and it also provides in-depth lifestyle assessments (£199) using chest monitors for extra accuracy. We took this test – which involved wearing a monitor for three days – then spoke to Nigel Stockill, performance director at Firstbeat UK, about heart rate variability, what it can tell you about your stress levels, and the limitations of the stress measurements on fitness trackers.
What Is Heart Rate Variability?
Your heart rate variability (HRV) is tracked by measuring the time between heartbeats – your heart rate rhythm. Even if your heart rate remains pretty constant, your heart rate rhythm can vary, and this shows whether or not your body is in a stressed state.
“When you look at heart rhythm rather than heart rate, we show a slightly different rhythm when we’re in relax and rest mode,” says Stockill. “What we typically see is a very gradual speeding up of our heart rate when we breathe in and a slowing down of our heart rate when we breathe out. This is when our body is undistracted by external or internal forces. It’s in a perfect state for recovery – it’s not busy digesting, or detoxifying alcohol, or dealing with a rapid rise in our cortisol release.”
In the Firstbeat lifestyle assessment these rest periods are marked in green. This shows a higher level of HRV, which indicates the body is relaxed and recovering.
“It’s a sign of less stress. If we go into fight-or-flight mode, it doesn’t mean we’re stressed per se, it just means we’re in that ready state. What happens then is that our body stops the speeding up and slowing down of heart rate and it becomes regular. You’re in a state of readiness – ready to pounce, or run, or whatever.”
The lifestyle assessment plots your HRV over the three-day period. The hope is that you see a good balance of green – where your HRV is high and your body is in a resting mode – and red – where your HRV is low and you’re in a more ready state. If you’re in the red all day and don’t get much green at night, over time you’re going to get more and more run down.
What Causes Low HRV?
You don’t need to feel stressed for your HRV to fall; it’s caused by anything that gets your body going. And the less fit you are, the longer it can take to get back into a resting state.
“It can be just meeting someone,” says Stockill. “It doesn’t mean you’re stressed, but it does mean you spark a little bit of adrenaline, of cortisol. Some people are a lot better at switching that off, whereas others, perhaps because their cardiorespiratory fitness isn’t that good, are not able to regulate that. It tends to be when they’re awake they’re always in red, and they’re only in green in the latter stages of sleep.”
Bodily functions are also key factors, such as digesting food or dealing with alcohol, which both put you into the red zone. This means drinking alcohol in the evening can affect how much you recover during your sleep.
“When the body detects alcohol it sees it as a poison or toxin, so it prioritises detoxifying that alcohol before it can relax. So if it was just one unit you can see that within one hour it detoxifies that, so you can relax quite quickly afterwards. For a lot of people alcohol may be used consciously or subconsciously as a sleep aid, therefore any alcohol that’s still in the system as you go to sleep will delay the onset of green, to the tune of an hour per unit of alcohol in the system.”
Other factors that keep stop you from going into a relaxed state include temperature and exposure to blue light, so staying away from screens last thing before bed and ensuring your bedroom is the right temperature for you are both important.
How Accurate Are HRV Measurements On Fitness Trackers?
One of the key differences with the lifestyle assessment is that an expert talks you through the results and gives you goals to help improve your recovery, rather than the simple stats provided by a wearable. However, perhaps even more important is the difference in accuracy between the chest monitor and a wrist wearable.
“They [wrist-worn trackers] use radial pulse, which isn’t bad compared with five years ago, but it’s obviously not as accurate as [chest straps],” says Stockill. “I wouldn’t be doing any lifestyle interventions based on those [wrist-worn trackers], but they do give you a bit of a guide to time asleep and that kind of thing.
“The difference between being in the red and in the green is so minuscule that if you’re trying to do that from what’s effectively blood flow estimate, you’re always going to be guessing a little bit.”
You can use a wrist-worn tracker to identify trends in your HRV that might highlight how factors like alcohol affect your stress levels, but if it’s something that you’re considering using to shape your lifestyle, a more accurate test is worthwhile. You can get the Firstbeat lifestyle assessment through experts like physios, personal trainers and nutritionists.