Not All Stress Is Bad. Here's How to Measure the Good Kind.

We tend to talk about stress the way we talk about cholesterol: as if all of it is bad, and less is always better. That is not what the science says. The truth is that your body needs a certain amount of well-dosed stress to stay strong, and the problem is not stress itself, but the wrong kind, in the wrong dose, for too long.

The interesting part is that we can now measure the difference. A single cheap biomarker, tracked on a ring or a watch you already own, gives you a surprisingly accurate read on how well your nervous system is coping with the total load in your life. Rugby teams use it. NBA franchises use it. Intensive care doctors use it to predict which patient is about to deteriorate. And you can use it to decide whether today is a day to push, or a day to recover.

That biomarker is heart rate variability (HRV).

Eustress, Distress, and the Idea of Hormesis

In 1936, Hans Selye coined the term "stress" to describe the body's response to any demand placed on it. He later distinguished between "eustress" (good stress) and "distress" (bad stress). Modern science has refined this into the concept of hormesis: the idea that a dose of a stressor too small to damage you, but large enough to provoke a response, makes you stronger.

This is how exercise works. A bout of running damages muscle fibres and causes oxidative stress. If you recover, you adapt: mitochondrial biogenesis, better antioxidant capacity, stronger cardiovascular system. If you don't recover, or you never stop, you accumulate damage.

The same principle underlies fasting (short, moderate food restriction activates cellular repair pathways like autophagy via mTOR and AMPK), cold exposure (norepinephrine spikes, brown fat activation), heat exposure (heat shock proteins, sauna-associated cardiovascular benefits per the Laukkanen Finnish cohort studies), and moderate psychological challenge (better stress resilience, anti-fragility).

The unifying feature is dose and recovery. Short, sharp, occasional stressors followed by full recovery produce adaptation. Low-grade, continuous stressors with no recovery produce wear and tear. The same biological systems that are built up by the first are worn down by the second.

So the question is not "how much stress is in my life?" The question is "how much of it is recoverable, and am I actually recovering?"

What HRV Actually Is

Your heart does not beat like a metronome. Even at rest, the interval between consecutive beats varies, by tens of milliseconds, beat to beat. This variation is heart rate variability.

HRV reflects the balance between the two branches of your autonomic nervous system: the sympathetic (fight-or-flight) branch, which accelerates the heart and suppresses variability, and the parasympathetic (rest-and-digest) branch, delivered mainly by the vagus nerve, which slows the heart and increases variability. A healthy, well-recovered nervous system produces high variability. A chronically stressed, under-recovered, or acutely ill nervous system produces low variability.

HRV is usually expressed in milliseconds, and the most commonly reported metric is RMSSD (root mean square of successive differences), which is sensitive to parasympathetic (vagal) activity and is robust enough to be measured by consumer wearables. Frequency-domain measures (like HF power and LF/HF ratio) give additional detail in clinical settings.

The key principle: absolute HRV values vary enormously between people (by age, sex, fitness, body habitus, genetics). Your own HRV compared to yours yesterday, last week, last month is far more informative than your HRV compared to someone else's.

How Elite Sport Uses HRV

In professional sport, HRV is now one of the standard daily readiness metrics alongside wellness questionnaires and sleep tracking. A 2013 Sports Medicine review by Plews and colleagues established the modern framework: daily morning HRV, averaged over 7 days, tracked as a moving average.

A stable or rising 7-day rolling HRV means the athlete is adapting well to training and can be pushed. A sustained drop, particularly if accompanied by elevated resting heart rate or a low wellness score, is a signal of either under-recovery, illness, or excess training load. In that state, high-intensity training risks injury, plateau, or overtraining syndrome.

This is how modern teams decide who starts, who rests, and who gets flagged to the medical staff before a hamstring tear or a viral illness shows up clinically. Australian Football League clubs, professional rugby union squads (including several Super Rugby and Premiership teams), the New Zealand All Blacks, numerous NBA franchises, and English Premier League clubs use HRV-guided load management. A 2024 Journal of Sports Sciences study on AFL players demonstrated that HRV tracked with internal and external training load over a full season, confirming it as a useful recovery marker in collision sports.

A 2024 Sensors narrative review by Moharrer and colleagues of mobile-device HRV monitoring in athletes found that routine near-daily measurement, with interpretation based on weekly averages and coefficient of variation rather than single-day readings, consistently outperformed predefined training plans for improving fitness adaptations.

How Hospitals Use HRV

In the hospital, HRV has a different and equally important role. In critical illness, sepsis, and severe infection, the autonomic nervous system decouples, a process that reduces HRV sharply, often before other vital signs deteriorate.

A 2018 systematic review in Critical Care found that reduced HRV in ICU patients was consistently associated with increased mortality, multi-organ dysfunction, and longer ICU stays. A 2024 Scientific Reports study on COVID-19 pneumonia patients confirmed that HRV on admission predicted ICU mortality independently of severity scores.

In paediatrics, continuous HRV monitoring is used in the form of the HeRO monitor in neonatal intensive care. Moorman and colleagues, in their 2011 Journal of Pediatrics randomised trial, showed that displaying a heart rate characteristics score (essentially a form of HRV) reduced mortality in very low birth weight infants by around 20 percent by allowing doctors to detect early sepsis hours before it became clinically obvious.

The principle is the same across sport and medicine: HRV is a window onto autonomic reserve. When your reserve is low, you are either doing too much, recovering too little, or fighting something you don't yet feel.

Why This Matters for Longevity

You are not a rugby player or an ICU patient. Why should you care?

Because your HRV, averaged over time, is one of the single strongest predictors of cardiovascular mortality in adults. The Framingham Heart Study (Tsuji et al, Circulation 1996) showed that lower HRV was associated with significantly higher all-cause mortality over two decades. Subsequent cohorts, including the ARIC study and the Rotterdam study, have confirmed this.

A 2020 Frontiers in Psychiatry meta-analysis by Jarczok and colleagues, pooling more than 50,000 participants, found that low HRV was associated with a 40 to 50 percent higher risk of all-cause mortality, after controlling for traditional risk factors. Low HRV is also linked to depression, anxiety, type 2 diabetes, inflammation (elevated IL-6 and CRP), and impaired cognitive function.

Higher HRV, by contrast, is associated with better mental health, better emotional regulation, better cardiovascular fitness, and slower biological ageing.

How to Measure HRV Yourself

The Gold Standard: Morning Orthostatic or Supine

For the most accurate tracking, measure HRV at the same time every day, ideally within a few minutes of waking, before you get out of bed, before coffee, in a supine (lying) position for 2 to 5 minutes. A chest strap (Polar H10, Garmin HRM-Pro) paired with a free app (Elite HRV, HRV4Training, Kubios HRV) is the most accurate consumer setup.

The Practical Standard: A Wearable

For most people, a ring (Oura) or a watch (WHOOP, Garmin, Apple Watch) measuring HRV overnight is good enough. These devices take multiple readings during sleep, filter out artefacts, and give you a daily nocturnal HRV score. It is less clinically precise than morning supine measurement, but far easier to adhere to, and behavioural adherence is what actually matters.

What to Track

Ignore the absolute number. Your baseline range is what matters. Over 2 to 3 weeks of measurement, you will see your personal normal range (for most adults, somewhere between 20 and 100 ms RMSSD). Then track the trend. Look for:

A sustained drop over 3+ days: consider whether you are under-sleeping, over-training, drinking too much, fighting an illness, or in a stressful life period. A sudden single-day drop often reflects last night's alcohol, a late meal, or poor sleep, usually not worth acting on by itself. A gradual upward trend with training: your system is adapting well. A chronic low baseline: worth investigating. Rule out subclinical cardiovascular disease, untreated sleep apnoea, depression, and chronic inflammation.

How to Improve Your HRV

The interventions with the strongest evidence are largely the same ones that improve everything else:

Sleep. The single largest acute modifier. Seven to nine hours with a consistent schedule.

Zone 2 cardio. Low-intensity, conversational aerobic training (150 to 300 minutes per week) improves vagal tone over 8 to 12 weeks. Running, cycling, swimming, brisk walking uphill all count.

Strength training. Two sessions a week. Increases HRV modestly and improves metabolic health.

Breathwork and meditation. Slow-breathing practice (around 6 breaths per minute) acutely raises HRV and, over weeks, raises baseline. Mindfulness-based stress reduction trials consistently show HRV improvement.

Cold exposure and sauna. Short, repeated cold plunges or saunas improve HRV in some people, though individual responses vary. The Laukkanen Finnish cohort data shows sauna frequency inversely correlates with cardiovascular mortality.

Reduce alcohol. Alcohol is one of the most potent acute HRV suppressors. A single evening of drinking can flatten your HRV for 2 to 3 days.

Manage sympathetic drivers. Work stress, caffeine overuse, poor sleep hygiene, chronic inflammation. Tackle the inputs.

A Word of Caution

HRV data can be misleading if used without context. Single-day readings are noisy. A hot environment, a big meal, a late workout, a restless sleep, or a new medication can all shift your HRV. Read weekly trends, not single days. And don't become so anxious about your HRV score that your anxiety itself suppresses your HRV. I have seen this in patients. Orthosomnia with a twist.

HRV is also not a diagnostic tool. It is a readiness marker. A low score is a reason to look at the likely drivers; it is not, in itself, a diagnosis.

The Bottom Line

Stress is essential to being alive and to staying young. What makes it harmful is when the system no longer has the room to recover from it. HRV is the cleanest, cheapest, most accessible way we have today to see that recovery capacity in real time.

Track it. Don't obsess over it. Use it the way elite sport does: to pick the right day to push, and the right day to rest. Over months and years, you will accumulate vastly more high-quality work (whether in the gym, the office, or life) than someone who ignores the signal and just grinds.

Your autonomic nervous system is talking to you, between every heartbeat. It is worth listening.

References

1. Shaffer F, Ginsberg JP. An Overview of Heart Rate Variability Metrics and Norms. Frontiers in Public Health. 2017;5:258. DOI: 10.3389/fpubh.2017.00258
2. Plews DJ, Laursen PB, Stanley J, Kilding AE, Buchheit M. Training adaptation and heart rate variability in elite endurance athletes. Sports Medicine. 2013;43(9):773-781. DOI: 10.1007/s40279-013-0071-8
3. Moorman JR, Carlo WA, Kattwinkel J, et al. Mortality reduction by heart rate characteristic monitoring in very low birth weight neonates: a randomized trial. Journal of Pediatrics. 2011;159(6):900-906.e1. DOI: 10.1016/j.jpeds.2011.06.044
4. Tsuji H, Larson MG, Venditti FJ Jr, et al. Impact of reduced heart rate variability on risk for cardiac events: the Framingham Heart Study. Circulation. 1996;94(11):2850-2855. DOI: 10.1161/01.CIR.94.11.2850
5. Jarczok MN, Weimer K, Braun C, et al. Heart rate variability in the prediction of mortality: A systematic review and meta-analysis of healthy and patient populations. Neuroscience & Biobehavioral Reviews. 2022;143:104907. DOI: 10.1016/j.neubiorev.2022.104907
6. de Castilho FM, Ribeiro ALP, Nobre V, Barros G, de Sousa MR. Heart rate variability as predictor of mortality in sepsis: a systematic review. PLOS One. 2018;13(9):e0203487. DOI: 10.1371/journal.pone.0203487
7. Laukkanen T, Khan H, Zaccardi F, Laukkanen JA. Association between sauna bathing and fatal cardiovascular and all-cause mortality events. JAMA Internal Medicine. 2015;175(4):542-548. DOI: 10.1001/jamainternmed.2014.8187
8. Bellenger CR, Fuller JT, Thomson RL, Davison K, Robertson EY, Buckley JD. Monitoring Athletic Training Status Through Autonomic Heart Rate Regulation: A Systematic Review and Meta-Analysis. Sports Medicine. 2016;46(10):1461-1486. DOI: 10.1007/s40279-016-0484-2
9. Kivimäki M, Bartolomucci A, Kawachi I. The multiple roles of life stress in metabolic disorders. Nature Reviews Endocrinology. 2023;19(1):10-27. DOI: 10.1038/s41574-022-00746-8
10. Calabrese EJ, Mattson MP. How does hormesis impact biology, toxicology, and medicine? npj Aging and Mechanisms of Disease. 2017;3:13. DOI: 10.1038/s41514-017-0013-z