Stress and the Tactical Athlete: The Physiology, the Load You're Actually Carrying, and How to Manage It

What stress actually is in physiological terms. Why the body treats a bergen carry, a disciplinary interview and a poor night's sleep as the same signal, and the practices that keep the tank replenishable over years, not months.

Here is the BLUF version of this article you can act on tomorrow.

Stress is any input that pulls your body away from its normal internal state. Doesn't matter what caused it. Heavy training, a broken night, an argument at home, a caloric deficit, a disciplinary interview, a live contact. The body treats them as the same signal.

Total stress is additive. This is why the training block that felt fine last month is breaking you down now. The training hasn't changed. All the rest of the load has.

To stay ahead of it: monitor a small number of markers (morning HRV, resting heart rate, sleep quality, mood, session RPE). Take one or two clear evenings off every week. Breathe slowly through the nose for 5-10 minutes daily. Protect a consistent sleep-wake time. And when life stress climbs, reduce training load rather than push through.

The best predictor of long-term performance isn't how hard you can push. It's how well you recover from what you did yesterday.

That's the working answer. The rest of this article explains what's actually happening under the bonnet, why the modern concept of allostatic load has replaced the older overtraining framework, how to measure your stress state honestly, and the practices with real evidence behind them.

Homeostasis: the internal balance the body defends

To stay alive, the body has to keep a range of internal variables inside narrow parameters. Body temperature. Blood glucose. Blood pressure. Blood pH. Hormonal balance. Hydration.

Fall too far outside those parameters in any single variable and function starts to break down. Fall much further and death follows. The process of holding those variables inside their operating range is called homeostasis.

Homeostasis isn't a passive state. It's an active defence.

The body's constantly measuring, adjusting, correcting. Every mouthful of food, every heartbeat, every breath, every muscle contraction is part of that defence. It runs whether you notice it or not. And it takes precedence over almost everything else the body is asked to do.

This matters because it explains why stress, which is anything that pulls you away from homeostasis, is such a fundamental signal.

Stress: what it means physiologically

Stress, in physiological terms, is any input that disrupts homeostasis and forces the body to respond. That definition is broader than the everyday use of the word.

It includes emotional and psychological stress. But it also includes physical training. Heat. Cold. Illness. Sleep loss. Caloric deficit. Dehydration. Injury. All of them push the body off its baseline. All of them get the same core response.

The body's answer to any and all of these stressors is called the stress response. Its purpose is to mobilise energy and resources to deal with the disruption. Its structure is largely the same regardless of source.

Here's one of the most useful things to understand about stress: it's largely non-specific.

Your body treats a heavy lift session, a stressful phone call, a night on stag, a rough ration pack week, and a live contact as functionally similar events. Different intensities. Different durations. Same broad cascade.

The stress response: what actually happens under the bonnet

Two parallel systems drive the stress response. Both start in the brain. Both are automatic.

The first is the sympathetic nervous system. Within seconds of a stressor being registered, sympathetic activation drives the release of adrenaline (epinephrine) and noradrenaline (norepinephrine) from the adrenal medulla. Heart rate climbs. Blood pressure rises. Airways open. Pupils dilate. Blood is redirected away from the gut and toward the working muscles. Attention narrows. Reaction time sharpens.

This is the fight-or-flight response. It's optimised for one purpose: getting you through the next few seconds or minutes.

The second is the HPA axis: the hypothalamic-pituitary-adrenal axis. Within minutes to hours, the hypothalamus signals the pituitary, which signals the adrenal cortex, which releases cortisol. Cortisol has a longer time course than adrenaline and does different work. It mobilises glucose from the liver. Dampens inflammation. Suppresses non-essential functions like digestion, reproduction and immune activity. Shifts the body's metabolic priorities toward dealing with the demand at hand.

Both systems are designed to switch on hard, do their job, and then switch off. Once the stressor has passed, the parasympathetic nervous system takes over and returns the body to baseline. Heart rate drops. Digestion resumes. Immune function reactivates. Cortisol falls.

This return to baseline is where recovery, tissue repair, memory consolidation and adaptation actually happen.

The stress response isn't the problem. The failure to fully switch it off is.

 

Allostasis and allostatic load: the modern framework

Allostasis is the process the body uses to 'actively maintain homeostasis under a changing environment'.

Step out into the cold and your blood vessels constrict, your metabolic rate rises, you shiver. That's allostasis in action. The moment-to-moment adjustments the body makes to keep the internal environment stable while the external one isn't.

Then we have the concept of allostatic load.

Introduced by McEwen and colleagues in the late 1990s, allostatic load is now the dominant model for understanding chronic stress. It's the cumulative wear-and-tear on the body from repeated or sustained activation of the stress response.

Where allostasis is the adjustment, allostatic load is the cost of making that adjustment over and over again.

This is the concept that matters most to tactical athletes. Allostatic load builds slowly. It accumulates from training, sleep loss, caloric deficit, life stressors, environmental exposure, and emotional demand. The individual stressors are usually manageable in isolation.

What breaks all of us down is the accumulation of stress.

Recent military research has confirmed that higher allostatic load in candidates going through arduous training is associated with worse physical performance and higher rates of overuse musculoskeletal injury (Coombs et al. 2025). The same body that can handle a 20-mile tab can be broken by that same tab stacked on top of two poor nights of sleep, a family crisis, and a 500-calorie deficit - it's rarely just one thing.

Modern research suggests you can quantify this somewhat. Allostatic load indices combine multiple biomarkers (cortisol, inflammatory markers, cardiovascular indicators, metabolic markers) to give a composite picture of the total physiological cost of accumulated stress.

You don't need laboratory access to work with the concept. What you do need is the framework it points to: your total stress load is the sum of every input. Training's only one of them.

Why total stress is additive

The single most useful practical implication of the stress model is that everything counts toward the same bucket. Training. Sleep. Nutrition. Work demands. Relationships. Financial pressure. Environmental stressors.

The body doesn't have a separate accounting system for each. It has one stress response, one recovery capacity, and one running total of what's been asked of it.

For the tactical athlete, this is where most planning falls apart.

The training block that felt achievable in the calm month before deployment becomes impossible during the deployment itself. Not because the training changed. Because the total load did. The programme that you were flying through with no issues in the winter build phase, feels like it's a grind and falls apart during the summer when you're in the middle of moving house. The candidate who was flying through the plan during a stable period, at the unit, physically crumbles when the relationship with their partner breaks down. Same training. Different stress load.

Two practical consequences follow.

• Programme to the total load, not just the training load. Weeks with high life stress warrant reduced training stress, not more. The athlete who insists on maintaining programme volume during a personal crisis is usually the athlete who breaks two weeks later.

• The body can't tell the difference between stressors. A poor night's sleep and a hard interval session both leave you with less capacity in the tank tomorrow. Which is why programming that ignores sleep, life stress or nutrition status will fail more often than it succeeds.

None of this means you stop training when life gets hard. It means you calibrate the training to what your body can actually absorb, not what your ambition is asking of it.

Concurrent training makes this easier because it gives you dials, volume, intensity, session type, that can be turned independently. The Concurrent Training article on the website makes that case in more detail.

Mental stress: why the body treats it as physical

Even thinking about an advance to contact reads to the body as a physical event, even before you're physically doing anything.

Heart rate elevated. Breathing rapid. Acutely aware of every noise and movement. Adrenaline running through your system. All of this while you're just imagining about just walking down a track.

The reason your body does this is that mental stress is real stress. The brain doesn't draw a clean line between the anticipation or visualisation of a threat and the threat itself. If it did, you'd be caught flat-footed the moment the round cracked overhead. The stress response is designed to prime you before the contact, not after.

In the moment of contact, it's protective.

Sustained across weeks or months, though, mental stress produces the same physiological load as sustained physical stress. Cortisol elevated. Sleep disturbed. Recovery impaired. Immune function is suppressed.

This matters because tactical athletes tend to underestimate the cost of mental and emotional stress in their training planning. Physical training is measurable and visible. Mental load isn't. But the body responds to both the same way.

A month of high operational anxiety loads the system as heavily as a month of hard training. Programme accordingly.

The chronic stress problem

Acute stress isn't the problem. Sustained, unresolved stress is.

When cortisol stays elevated over weeks and months, when the sympathetic nervous system dominates and the parasympathetic system never fully takes back over, the body starts to pay costs it isn't really set up to handle.

• Immune function is suppressed. The frequent minor illnesses candidates get during selection prep are rarely random. They cluster around periods of high total load. Especially noticeable after multiple night/weeks of wrecked sleep.

• Sleep architecture is disrupted. Sustained cortisol elevation shortens deep sleep and disrupts REM. Even a full 8 hours in bed produces a shallow, unrefreshing rest. This creates a feedback loop: stress worsens sleep, increased caffeine use worsens sleep, and poor sleep raises stress.

• Recovery from training slows. The same programme that produced adaptation last month now produces hideous fatigue. The volume you used to handle now leaves you flat.

• Injury risk rises. Chronic stress reduces tissue quality and impairs neuromuscular coordination. The link between allostatic load and overuse injury in military populations is well documented.

• Mood and cognition suffer. Persistent elevated cortisol is associated with reduced hippocampal volume, impaired memory consolidation, and heightened emotional reactivity. Selection candidates in the deepest fatigue often feel like a foggy, short-tempered version of themselves that they don't recognise.

None of this is inevitable. It's what happens when the accumulation gets ignored.

The task is to notice it as early as possible and act on it.

How to measure your stress load

Guessing at your stress state is unreliable.

Even highly experienced athletes chronically underestimate how loaded they are. The sympathetic dominance of accumulated stress itself masks the fatigue underneath. You feel wired. You interpret that as fine. You aren't.

The remedy is to track a small number of objective and subjective markers, and treat trends in them as signal.

Table 1. Practical markers of stress load for the tactical athlete. Track a small number daily. Watch trends across a week, not single-day readings.

Morning HRV is now available from most wearables (Whoop, Garmin, Oura, Apple Watch). The absolute number varies enormously between individuals and doesn't mean much in isolation but the overall trend against your rolling weekly average does.

A single low day means nothing (or a single high day). Three or four consecutive low days is a signal worth paying attention to (Nuuttila et al. 2024).

Resting heart rate, taken first thing in the morning before you get out of bed if you can, is a cheap and reliable index of accumulated stress. A resting HR consistently 7-10 bpm above your normal baseline across several days is one of the earliest markers of overload.

But, subjective markers matter just as much as objective ones.

Session RPE rising while the objective work stays the same is a reliable early warning. So is disrupted sleep, particularly the wired-but-tired pattern of waking at 03:00 or 04:00 with a racing mind. Honest tracking of three or four things beats any single biomarker in isolation.

How to manage stress: the practices with real evidence

Stress management has become an industry of the vague.

What follows are the interventions that show up consistently in the evidence base.

Table 2. Evidence-backed stress management practices for tactical athletes. None of them are exotic. All of them work when applied consistently.

Two absences worth naming.

Long supplement lists don't appear in this table. The evidence for adaptogens, nootropics, magnesium, ashwagandha and other stress-marketed products is weak, mixed, or in most cases actively contradictory. Ashwagandha has some short-term cortisol data behind it, but the trials are small and the effects modest. Magnesium is useful only if you're deficient. None of them come close to the impact of consistent sleep, breath work, and time genuinely off.

The other absence is intense exercise as a stress reliever. In the right dose and phase, it can help for sure, but stacked on top of an already-elevated stress load, more hard exercise usually deepens that hole you're trying to climb out of rather than filling it.

Programming around stress

Once you can measure it, the practical question becomes what to do with the information. Three rules that hold for most tactical athletes.

• Reduce training load when life stress climbs. Drop weekly volume by 20-40% during high-stress periods (deployment, workups, family emergencies, house moves, legal things, work upheaval). Keep intensity on some sessions to maintain the training signal. Cut total minutes.

• Prioritise Zone 2 aerobic work under stress. It builds capacity without producing much additional allostatic load. High-intensity intervals stacked on top of accumulated stress are one of the fastest ways to break yourself.

• Rebuild before pushing again. A week of consolidation after a high-stress period isn't lost training. It's the training. Adaptation happens in recovery, not in the session that caused the stress.

None of this is glamorous. All of it works better than the alternative, which is grinding harder through periods of high accumulated load in the hope that willpower will substitute for physiological capacity - It rarely does.

Bottom line

Stress isn't the enemy. It's the signal your body uses to change.

What the body can't handle is stress that never gets switched off, stress that stacks faster than it can be recovered from, or stress that gets applied indiscriminately without regard for what else is already in the tank.

The tactical athletes who last the longest and perform the highest aren't necessarily the ones who tolerate the most stress. They're the ones who manage the total load, measure their state honestly, and back off when the numbers tell them to.

This is unsexy and un-social-media-worthy. It's also what works.

Five practical takeaways.

• Track morning HRV, resting heart rate, sleep quality, mood and session RPE. Watch trends across a week.

• Treat total stress as one bucket. Life stress and training stress fill it from the same tap.

• Protect deliberate parasympathetic time: slow breathing, walks outside, evenings genuinely off.

• Reduce training load when life stress climbs. Zone 2 is your friend under high accumulated stress.

• Sleep is the single largest lever on all of the above. Protect it accordingly.

Stress and recovery: frequently asked questions

What's the difference between good stress and bad stress?

Physiologically, there isn't much of a difference in the acute response. Both trigger the same cascade.

The distinction is in duration and recovery. A hard training session is stress that produces adaptation because it's followed by recovery. Chronic elevated stress becomes damaging when the recovery phase never fully happens. The stress itself isn't the problem. The failure to switch it off is.

What's allostatic load in plain terms?

The cumulative wear-and-tear on the body from carrying stress for extended periods. Every stressor asks the body to make an adjustment. Every adjustment costs something.

Allostatic load is the running total of those costs. It builds slowly, mostly out of sight, and becomes visible through fatigue, illness, injury, mood changes and stalled progress.

Do I need a wearable to track my stress?

No, but they help.

A resting heart rate taken first thing in the morning is free and reliable. Subjective tracking of sleep quality, session RPE and mood catches most of what matters. Wearables add HRV, which is a useful additional index but not essential. The best system is the one you actually use consistently.

Is cortisol bad?

No. Cortisol is essential. Without it you wouldn't wake up, get out of bed, handle a stressor, or metabolise glucose.

The problem isn't cortisol itself. The problem is cortisol that stays elevated when it should have fallen. Acute cortisol elevation is adaptive. Chronic elevation is what drives the downstream effects on sleep, immunity and mood.

What's HRV and what should I do with the number?

Heart rate variability is the variation in time between successive heartbeats. It reflects the balance between the sympathetic and parasympathetic nervous systems. Higher HRV generally indicates better recovery and parasympathetic dominance.

The absolute number varies enormously between individuals and isn't meaningful in isolation. What matters is the trend against your own rolling average. Three or four consecutive days significantly below your baseline is a signal that accumulated stress is exceeding your recovery capacity.

Can breathwork actually change anything, or is it just relaxation?

It genuinely changes autonomic state.

Slow nasal breathing at 5-6 breaths per minute activates the vagus nerve, which is the main parasympathetic pathway. Over weeks, this produces measurable increases in resting HRV and reductions in resting heart rate and cortisol. It isn't a mystical practice. It's a direct intervention on the autonomic nervous system, and it costs nothing.

How much should I reduce training when I'm under high life stress?

A useful starting point is a 20-40% cut in weekly volume, holding intensity on one or two sessions to keep the training signal alive. The specific number depends on what's driving the stress, how long it's likely to last, and how loaded you already were.

The general principle is that maintaining performance under high stress is a fantasy. The honest goal is to preserve the base and be ready to build again when the stress lifts.

Should tactical athletes meditate?

If it fits your life, it earns its place.

The evidence for meditation on cortisol, HRV and self-reported stress is real but modest. The bigger win, for most people, is the deliberate parasympathetic downtime that meditation forces. If you can achieve the same effect from a walk outside, a slow breathing session, or 20 minutes without a phone, the mechanism is the same. The label is optional.

What about cold exposure and sauna for stress?

Both work, in the right context.

Regular sauna use is associated with improved cardiovascular markers and reduced all-cause mortality (Laukkanen et al. Finnish cohort data). Cold exposure improves resilience to acute stress via repeated controlled hormesis. Both are low-dose stressors themselves, though, and stacking them on top of a fully-loaded stress state can worsen rather than improve recovery. Use them during periods of low-to-moderate accumulated stress. Not in the middle of an already-crushing block.

Further reading

If you want to go deeper into the qualities that protect against accumulated stress and improve recovery, the following articles on the Stoic Conditioning site walk through each piece in detail:

• Nutrition for Tactical Athletes - why chronic under-fuelling is one of the largest stress inputs, and how to eat for the phase you're in.

• Slow Down to Speed Up - the zone 2 case, which sits at the centre of stress-conscious aerobic training.

• How to Train for Sleep Deprivation - what you can and can't train for when sleep goes.

References

Coombs SC, et al. Association of allostatic load measured by allostatic load index on physical performance and psychological responses during arduous military training. Physiological Reports, 2025.

Coombs SC, et al. Advancing the allostatic load model in military training research: from theory to application. Frontiers in Physiology, 2025.

McEwen BS. Stress, adaptation, and disease. Allostasis and allostatic load. Annals of the New York Academy of Sciences, 840:33-44, 1998.

McEwen BS, Wingfield JC. The concept of allostasis in biology and biomedicine. Hormones and Behaviour, 43(1):2-15, 2003.

Nuuttila O-P, Nummela A, Kyrolainen H, et al. Monitoring training adaptation and recovery status in athletes using heart rate variability via mobile devices: a narrative review. Sensors, 2024.

Manzar MD, et al. Heart rate variability applications in strength and conditioning: a narrative review. Journal of Functional Morphology and Kinesiology, 2024.

Laborde S, Mosley E, Thayer JF. Heart rate variability and cardiac vagal tone in psychophysiological research: recommendations for experiment planning, data analysis, and data reporting. Frontiers in Psychology, 8:213, 2017.

Milewski MD, Skaggs DL, Bishop GA, et al. Chronic lack of sleep is associated with increased sports injuries in adolescent athletes. Journal of Pediatric Orthopaedics, 34(2):129-133, 2014.

Sapolsky RM. Why Zebras Don't Get Ulcers (3rd ed.). Henry Holt, 2004. (Accessible book-length treatment of the physiology in this article.)

Selye H. The stress of life. McGraw-Hill, 1956. (Original formulation of the general adaptation syndrome.)