How To Measure Stress
In a recent article I explored the “biohacking” approach to stress management – by this I don't mean doing weird experiments on yourself, but the art and science of optimising your well-being and performance by measuring, tracking and optimising your biology. For biohacking stress, the principle would be to measure stress objectively so you can see what form it takes for you as an individual, and thus how and where to target your change strategies. Then you'd measure again as a means of tracking changes, and judging whether you've been successful.
So how do you measure stress, or the biology of stress?
First let me clarify what I mean by stress. There are two parts to stress: the stress trigger and the stress response. In this article I'm going to be talking about measuring the stress response, or the baseline state of your stress response systems. I'm not going to be talking about e.g. questionnaires for assessing stress triggers (the big ones are death of a spouse or child, divorce, getting sacked). I'm also focusing on objective measures of physiology, rather than questionnaires.
The biology of stress is complex, and the stress response is an orchestrated action involving a number of body systems. No single measure can capture the whole of the stress response. Rather, there are a number of different stress biomarkers, each emphasising a particular aspect of the stress response, and each having pros and cons.
I'm going to discuss seven indicators, all of which I use myself in my stress resilience coaching business. They are:
- Heart Rate Variability (HRV)
- EEG or brainwaves
- Breathing assessment via capnometry
- Adrenal assessment
- Skin conductance and skin temperature
- Sleep tracking
- Resting heart rate or passive heart rate
Let's look at these in more depth one by one.
Probably the most commonly used method of stress assessment is Heart Rate Variability or HRV analysis. HRV analysis is as you might guess: looking at patterns of change in heart rate over time, and in particular rhythms in the variation, at different time scales.
Heart rate is modulated by the Autonomic Nervous System (ANS) which has two branches:
- the sympathetic which drives fight-or-flight and raises heart rate, and
- the parasympathetic which drives the relaxation response, and slows heart rate.
Thus, HRV can tell us something about ANS functioning, and the balance between sympathetic and parasympathetic (the latter is a somewhat controversial question – it seems that HRV is a good biomarker for the parasympathetic but not really the sympathetic).
One potential pitfall of HRV as a stress biomarker is that the results are strongly influenced by breathing rate. Of course breathing rate is influenced by stress, but you can easily change your breathing rate for the duration of the assessment and thus end up with a distorted picture. I suspect a lot of people don't realise this.
There are a number of HRV assessment systems available, both professional and consumer. A good starting point is a low cost app called SweetBeat HRV from Sweetwater Health, which can work with low cost heart sensors.
HRV has also been shown to be a useful general health marker. It's also a powerful biofeedback parameter (associated with heart coherence – but be careful not to get HRV assessment and heart coherence mixed up – for HRV assessment, you want a normal everyday state rather than coherence).
EEG is a voltage measured from the scalp, and rhythms within the EEG are known as brainwaves. Particular frequency bands have names like alpha and theta, and the relative amounts of activity in each band correlate with mental and emotional state, to a degree.
In my practice I follow a system devised by Dr Paul Swingle, and described in his book “Biofeedback for the Brain”. His system looks at several markers within the EEG – too many to list here, but I will mention one or two to give you a flavour.
Theta to beta ratio at the back of the head – when this is low it suggests a busy, agitated mind, racing thoughts that are difficult to switch off. It's probably the most commonly found stress marker – though a lot of people show it and don't necessarily experience it as anxiety.
Left-right balance at the front of the head – imbalance tends to show up mood problems.
My experience of using this relatively simple EEG system is that it seems pretty accurate in the sense that it agrees with the person's experience. I like it in that it is more stable than say HRV – if you tried your best to relax for the duration of the assessment, it probably wouldn't make much difference to the results.
Here is a video of Dr Swingle describing his system:
EEG is a massively complex measurement and there are systems that go way beyond what Dr Swingle's system does (I'm talking about QEEG for those interested) but there's no space to discuss them here.
3. Breathing assessment with capnometry
A common facet of the stress response is a shift to faster, chest-based breathing and over-breathing. Over-breathing is a matter of degree, and mild cases are largely unrecognised. A capnometer can assess degree of over-breathing by measuring carbon dioxide in exhaled air. Over-breathing is an important topic that I've written about in other articles.
The capnometer is a powerful biofeedback device for optimal breathing training, and probably my favourite because it can deliver significant changes relatively easily.
4. Adrenal assessment
Yet another dimension of the stress response is hormonal. The best known stress hormones are adrenalin and cortisol. You probably know that adrenalin creates a rush of energy to help you deal with stress (it's part of “fight-or-flight”). Cortisol does something similar but it longer acting. For example it increases blood sugar so that cells have more fuel available.
When cortisol is chronically high it causes problems (it damages the brain, and the symptoms tend to be anxiety and over-arousal, feeling wired and hyper-vigilant). Cortisol can also drop too low – this tends to be seen in exhaustion and burn-out.
Cortisol has – or should have – a daily rhythm: it should be high in the morning when you get up (giving you energy) and drop over the course of the day so it's low at bedtime. Often this rhythm is lost in chronic stress.
To assess adrenal function you need a laboratory test. The one I favour uses dried urine samples, which are easy to collect and transport. You ideally need four or more samples from one day, so that you can see the daily rhythm.
Adrenal lab tests also typically look at another adrenal hormone, DHEA – but not adrenalin which is too short-lived to be meaningfully tested.
5. Skin conductance & skin temperature
These two measures have the advantage that they can be easily measured from the skin. Changes are driven by the sympathetic nervous system, mentioned earlier as a driver of the fight-or-flight response. Both are responsive in the short term, especially skin conductance, which makes them useful for demonstrating and learning about the stress response. Skin conductance, which is related to Galvanic Skin Response (GSR) and Electrodermal Response, is used in the famous polygraph or “lie detector” test. It can't really detect lies but it can detect the immediate spike in sympathetic activity which you'd expect from emotionally significant goings on (e.g. denying you committed murder).
Their disadvantage is that they're influenced by several other factors besides stress. This means that while relative changes are useful, absolute measurements are less clear-cut: you can't say a reading of X means a stress level of Y. For example, both skin conductance and skin temperature are influenced by the ambient room temperature.
6. Sleep Tracking
Sleep is such a fundamental part of life – if we don't get enough, or perhaps even more importantly sleep quality is lacking, then every aspect of physiological functioning is affected, and thus so is every aspect of health and well-being.
Ideally you would use a high-end EEG based sleep monitoring system to track sleep and sleep quality: optimal sleep means the right amount of sleep but also the right proportions of deep sleep, light sleep and REM sleep (the latter is related to dreaming and also to emotional stress).
These days you can buy numerous consumer devices for relatively low cost, that claim to track sleep, including at least one or two EEG based devices. Most of the others are based on heart rate and heart rate variability analysis. Some claim to differentiate light sleep, deep sleep and REM sleep.
I've tried some of them, and my impression is the heart rate devices aren't very accurate, but may be good enough for at least tracking changes.
7. Resting Heart Rate & Passive Heart Rate
I list passive heart rate because it's also reported by a lot of the sleep tracking consumer devices I've mentioned. Strictly speaking resting heart rate is measured once per day just before you wake up. I think it is a useful indicator of stress and overall fitness and health – the lower it is the better. But it's not ideal because again there are so many influences on it besides stress. An advantage is that many low-cost consumer devices can measure it.
There is no single indicator that captures everything worth knowing about the status of your stress systems. I've listed several markers, each having pros and cons. Ultimately the worth of any measure is in the decisions you can make on the basis of it: does the measurement suggest particular options and actions that will make a difference, and bring you closer to optimal functioning, over and above other options.
Again, all the markers discussed here are ones I've used myself either directly with clients, or I've advised clients on. If you think I might be able to help you with stress or anxiety please do get in touch.
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