The Stress Resilience Stack: Exercise and Sleep

The non-negotiable foundations

One of the most consistent patterns we see at A3 across hundreds of clients and thousands of data points is chronic stress. Not the acute, productive kind, but the low-grade, always-on kind that shows up in suppressed HRV, cortisol that never quite normalizes, and performance that degrades over time. We think of it as the unsung tax on high achievement. This series is a deep dive into a full stack of evidence-based interventions for doing something about it. As we said in Part 1, the research is clear: while there may not be a single magic bullet, multiple moderate interventions quickly add up.

Part 1 covered the entry point: breathing and mental practices. Those are free, portable, and surprisingly powerful. They're techniques you can use anywhere, anytime, with nothing but your own body and attention.

But here's the thing: those techniques work best when they're built on a solid foundation. You can do all the cyclic sighing and slow breathing you want, but if you're sleeping five hours a night or haven't broken a sweat in months, you're fighting an uphill battle. The breathing practices shift your nervous system in the moment; exercise and sleep determine what your nervous system's baseline looks like in the first place.

That's what this post is about: the non-negotiables. The load-bearing walls of stress resilience that everything else builds on.

Exercise and sleep aren't separate interventions. They're deeply interconnected. Exercise quality affects sleep architecture; sleep quality affects recovery, HRV, and your capacity to adapt to training stress. Get these right, and the other tools in your stack work better. Get them wrong, and you'll undercut the effects of nearly everything else.

The good news: the research here is robust, the protocols are well-established, and the interventions don't require exotic supplements or expensive devices. The bad news: there are no shortcuts. This is where consistency matters more than optimization.

Let's get into it.

Exercise

There's a paradox at the heart of exercise and stress: physical exertion is itself a stressor. It spikes cortisol, elevates heart rate, and temporarily suppresses immune function. Yet regular exercisers consistently show lower baseline cortisol, higher HRV, and better stress resilience than sedentary people.

The resolution is hormesis, the process by which controlled, recoverable stress makes you more resilient to stress in general. Exercise is a training stimulus for your entire stress-response system, not just your muscles. But the details matter. Different types of exercise produce different adaptations, and more isn't always better.

Zone 2 Cardio: Building Parasympathetic Reserve

If there's one type of exercise that earns the "non-negotiable" label for stress resilience, it's Zone 2 cardio—longer-duration aerobic work at 60-70% of your max heart rate (essentially, the pace at which you could still mostly carry on a conversation). Think jogging, cycling, swimming, or brisk walking—anything that keeps your heart rate elevated for 30-60 minutes straight.

The research here is robust. Meta-analyses show that consistent aerobic training produces significant increases in HRV metrics (RMSSD, high-frequency power, SDNN) over 8-12 weeks. Higher training frequencies and longer durations generally produce larger effects, at least when balanced against recovery needs. The sweet spot for most people: 30-60 minutes, 3-5 sessions per week.

Physiologically, Zone 2 training builds your parasympathetic reserve. Regular aerobic exercise increases vagal tone, the strength of your parasympathetic nervous system's influence on your heart. That means a lower resting heart rate, faster recovery from acute stressors, and a higher HRV baseline. You're expanding the capacity of your "rest and digest" system.

Even better, the effects aren't just cardiovascular. Aerobic exercise increases BDNF (brain-derived neurotrophic factor), which supports neuroplasticity and has antidepressant effects. It improves insulin sensitivity, reduces systemic inflammation, and enhances sleep quality. It builds the broadest base of adaptive benefits with the lowest recovery cost.

Before you hop in, one practical note: Zone 2 should feel easy. If you're gasping or can't hold a conversation, you've drifted into Zone 3 or higher. Especially if you're not a already a competitive endurance athlete, you probably need to start much slower than you'd think.

HIIT: Stress Inoculation

High-intensity interval training—short bursts of all-out effort followed by recovery periods—works through a different mechanism than Zone 2. Where Zone 2 builds your aerobic base gradually, HIIT is all about acute stress inoculation.

A typical HIIT session spikes cortisol by roughly 80% immediately post-exercise. Testosterone surges, too. But by 2-3 hours later, both hormones then drop well below baseline (with impressive effect sizes around d = -0.95 to -1.08 for cortisol in particular). Stress followed by super-compensation, also known as "hormesis," is what builds resilience over time.

Research shows that trained athletes display significantly lower cortisol responses to psychological stress compared to untrained individuals. Their systems have learned, through repeated exposure to controlled physical stress, to mount an appropriate response and then recover efficiently. HIIT trains this capacity directly.

That said, there's a ceiling. More than 2-3 HIIT sessions per week, without adequate recovery, will likely tip you into overtraining territory (further discussed below). The stress stops being hormetic, and starts becoming cumulative. If you're already dealing with high life stress (like most of our clients), piling on too much high-intensity training can easily backfire, adding to your total stress load rather than building resilience against it.

The practical recommendation: 2-3 HIIT sessions per week maximum, with at least one full rest day between sessions. If you're new to exercise or in a particularly stressful life period, start with Zone 2 only and add HIIT once your aerobic base is established or life has evened out.

Strength Training: Powerful, but Not Sufficient

Given that strength training is central to what we do at A3, we're adding this section to address the elephant in the room: while resistance training has real benefits for stress resilience, it's probably not enough on its own.

Research shows that strength training can reduce anxiety symptoms, with meta-analyses finding small-to-moderate effect sizes. It improves sleep quality, boosts self-efficacy, and produces favorable changes in cortisol patterns over time. Acute sessions spike cortisol similarly to HIIT, with the same hormetic recovery pattern. And the downstream benefits, like improved body composition, better insulin sensitivity, and increased functional capacity all contribute indirectly to stress resilience.

But there's still a real gap. Strength training doesn't build aerobic capacity or parasympathetic reserve the way Zone 2 cardio does. The HRV improvements from resistance training alone are modest compared to those from sustained aerobic or HIIT work. If you're only lifting and never doing cardio, you're leaving significant adaptations on the table.

Our take: strength training is essential for long-term health, performance, and longevity, and it's a core part of what we program for clients. But for stress resilience specifically, it works best as a complement to aerobic training, not a replacement for it. The ideal stack includes all of the above: Zone 2 for building parasympathetic reserve, strength training for the structural and metabolic benefits, and HIIT used sparingly for high-intensity stress inoculation.

Yoga: Meaningful Effects, Different Pathway

Yoga often gets dismissed as "not real exercise" by the intensity-focused crowd, which is a mistake. Meta-analyses show yoga produces large effect sizes for anxiety reduction. HRV improvements are documented across yoga styles, with increases in high-frequency power and decreases in the LF/HF ratio that indicate a shift toward parasympathetic dominance.

What makes yoga interesting is that it combines physical movement with breath regulation and attentional focus—essentially integrating the breathing practices from Part 1 with low-intensity exercise. For people who find pure cardio tedious or have physical limitations that preclude running or cycling, yoga offers a genuine alternative pathway to stress resilience.

The minimum effective dose appears to be once-weekly, 1-hour sessions, sustained for at least 8 weeks. Two to three sessions per week produces stronger effects. But it’s not a quick fix; like most stress interventions, the benefits compound with consistency.

Nature Walking: The Multiplier

We covered nature exposure in Part 1 as a mental practice, but it's worth revisiting here because combining exercise with nature amplifies the benefits of both.

Walking in nature reduces cortisol by 53%, compared to 37% for urban walking at the same intensity and duration. HRV shows a 104% increase in RMSSD during nature walks. A 50-minute nature walk decreased anxiety, reduced rumination, and improved working memory compared to an urban walk. Same physical activity, dramatically different physiological and psychological outcomes.

The practical implication: if you're choosing between a treadmill and a park, choose the park. If you're choosing between a gym and a trail, choose the trail. You're not sacrificing training quality, you're adding a multiplier. Even 15 minutes of walking in a green space produces measurable changes in stress markers.

Overtraining: The Warning Signs

More exercise isn’t always better. Overtraining syndrome is real, and it's especially insidious because the early symptoms—fatigue, irritability, poor sleep—look a lot like the very stress you're trying to address.

The clearest objective marker is HRV: a sustained decline over 3-4 weeks, despite adequate sleep and nutrition, is a red flag. Other warning signs include unexplained performance plateaus or declines, elevated resting heart rate, persistent fatigue that doesn't resolve with rest, sleep disturbances, mood deterioration, and increased susceptibility to illness.

Prevention is straightforward: build in deload weeks. Every 4-6 weeks, reduce training volume and intensity by 40-50%. This isn't lost progress, it's when adaptation actually consolidates. Monitor your morning HRV if you have a wearable (and remember that trends matter more than absolute numbers). And if you're going through a high-stress life period, consider temporarily reducing training load rather than pushing through.

The goal is to use exercise as a stress inoculator, not to add another source of unrecovered stress to your life.

Sleep

If exercise is where you start building stress resilience, sleep is where you actually lock it in. Every adaptation we just discussed (HRV improvements, hormetic recovery from training, parasympathetic gains) consolidates during sleep. Skimp on sleep, and you're not just tired, you're actively undercutting the impact of your exercise efforts, and degrading the systems that protect you from stress.

Sleep deprivation reliably increases cortisol, reduces HRV, impairs emotional regulation, and compromises immune function. One night of poor sleep is recoverable. But chronic sleep debt compounds. And the effects look remarkably similar to chronic stress itself, because physiologically, that's exactly what it is.

The challenge here is that most high achievers have become completely accustomed to a baseline of suboptimal sleep. Your brain on five or six hours starts to feel normal. Caffeine masks the subjective experience of tiredness. And because the decline is gradual, you lose the reference point for what "well-rested" even feels like. Still, the ugly truth is: you're not fine, you've just forgotten what fine is.

To get things fixed, here's what actually moves the needle.

Morning Light: Anchoring Your Circadian Rhythm

The single most underrated sleep intervention doesn't happen at night. It happens within the first 30-60 minutes after you wake up: exposure to bright light.

Morning light exposure advances your circadian phase, enhances the cortisol awakening response (which you want—it's what makes you alert in the morning), and sets up your melatonin secretion timing for the following night. Meta-analyses show substantial sleep improvement effect sizes (g = 0.39-0.47) from light exposure alone. That's a meaningful impact from something that's completely free.

The key is intensity. Outdoor sunlight delivers 10,000-100,000 lux on a bright day. Even overcast conditions provide 1,000-10,000 lux. Indoor lighting? It looks bright, because your eyes adapt so effectively, but it's typically just 100-500 lux, an order of magnitude too dim to make a difference.

The protocol is simple: 5-10 minutes of direct sunlight exposure within an hour of waking. Longer if it's overcast. Face toward the sun (though, obviously, not staring at it directly), ideally without sunglasses blocking the light from reaching your eyes. Phase shifts are measurable within 2-3 days of consistent practice.

If you work from home or have a flexible morning routine, this is one of the easiest wins in the entire stack: walk outside with your coffee, or eat breakfast by a window. If you’re headed to an office, try to slot sun time into the logistics of your commute. Either way, make light exposure part of your wake-up ritual, not something you stumble into by accident.

Bedroom Temperature: The Deep Sleep Lever

Your body needs to drop its core temperature by about 1-2°F to initiate and maintain deep sleep. Fight this process (with a warm bedroom, heavy blankets, or poor ventilation) and you'll selectively lose slow-wave sleep, the most restorative phase.

Large-scale studies confirm the optimal bedroom temperature for sleep is 60-67°F (15-19°C). A 10°C increase causes a 20% increase in the odds of insufficient sleep. The relationship is consistent and dose-dependent.

Practical fixes: turn down the thermostat (or, in the winter, open the window) at night, use breathable bedding, and consider a warm bath or shower 1-2 hours before bed. That last one is counterintuitive but well-supported. Warming your body before bed actually accelerates a subsequent core temperature drop, shortening sleep onset latency by approximately 36%. (I.e., you're not warming up to sleep warm, you're warming up to cool down faster.)

Similarly, there are dedicated cooling devices (like the Eight Sleep or ChiliPad) that actively regulate mattress temperature throughout the night. They're admittedly pricey, and the peer-reviewed evidence is limited, but our clients have consistently found them overwhelmingly helpful for deep sleep improvements (and their data backs it up). Worth considering if temperature is a clear issue for you, especially if you've already tried out the zero-cost room environment optimizations.

(And, on a related note: temperature is powerful beyond just overnight; we'll go deeper on interventions like cold exposure and sauna in Part 3.)

Caffeine: The Hidden Saboteur

Caffeine has a half-life of 5-6 hours. That means half the caffeine from your 2pm coffee is still circulating at 8pm. A quarter is still there at 2am.

What makes caffeine insidious is that it disrupts sleep architecture even when you feel like you're sleeping fine. You might fall asleep without trouble and stay asleep through the night, but the depth and quality of that sleep is compromised. Slow-wave sleep decreases. Sleep efficiency drops. HRV suffers. And because you don't feel awake at 3am, you don't connect the cause and effect.

The conservative recommendation is a caffeine curfew 8-10 hours before bed. If you hit the sack at 11pm, that means no caffeine after 1-3pm.

But add to that one more wrinkle: caffeine metabolism is hugely genetically variable. Genes like CYP1A2 determine how quickly you clear caffeine, and roughly half the population are "slow metabolizers" who may need even earlier cutoffs. If you've ever suspected caffeine affects you more than others, you're probably right. (That's one of the many reasons we do full genetic sequencing of all our clients.)

Alcohol: The Recovery Killer

This one hurts, as we love a well-mixed negroni or a good montepulciano. But alcohol is a sedative that fragments sleep architecture, suppresses REM throughout the night, and tanks HRV.

The data here is striking. OURA tracked over 600,000 user-nights and found that alcohol consumption was associated with a 15.6% mean decrease in HRV, 35 fewer minutes of sleep, and 6.8% lower sleep scores. This held even for moderate consumption—a glass or two of wine, not binge drinking.

Objectively, there's no "safe" dose of alcohol for optimal sleep quality. If you're optimizing for recovery and stress resilience, alcohol is working against you.

That said, we at A3 aren't teetotalers (just like we still sometimes eat less-than-healthful food, because it's delicious); we're all about crafting a balance between enjoyment and long-term health that works for you now and for the decades to come. If you do choose to drink, finishing 3-4+ hours before bed at least allows some metabolism before sleep, somewhat reducing negative impact. And post-drink nights are also a great time to check in on morning-after changes in HRV; that way, you'll have a clearer sense of the costs (and when they're still absolutely worth it).

Supplements: A Preview

Finally, we'll just mention briefly that there are a slew of evidence-based supplements that support sleep (e.g., melatonin [at much lower doses than most people think], magnesium glycinate and threonate, glycine, and others). Some of them have surprisingly strong research behind them. Still, we're saving the full breakdown for Part 4, where we'll cover all the ingestibles—from basic supplements to adaptogens to more exotic interventions—in one place. For now, the behavioral and environmental factors above are where to start. Fix the foundation before adding supplements on top.

Things to Try Today

We've covered a lot. Here's how to actually start:

If you're not doing regular cardio: Start with Zone 2. Thirty minutes at conversational pace, three times a week. Don't jump straight into HIIT—build the aerobic base first. This is the single highest-leverage exercise intervention for stress resilience.

If you're exercising but still feeling chronically stressed: Check your balance. Too much HIIT relative to Zone 2? Not enough recovery between sessions? Consider swapping one high-intensity session for a longer, easier effort, and watch your HRV trends over the following weeks.

If you want to amplify your existing routine: Move it outside. The same workout in a park or on a trail produces measurably better outcomes than the same workout in a gym. If outdoor training isn't practical, even a 15-minute walk in green space on rest days adds a real multiplier.

For immediate sleep improvement tonight: Drop your bedroom temperature to 65°F, or as low as you can tolerate. This is the fastest environmental fix for deep sleep. If you tend to run hot, try the warm shower trick 1-2 hours before bed to accelerate your core temperature drop.

Starting tomorrow morning: Ten minutes of sunlight within 30 minutes of waking. No sunglasses. Make it part of your routine: walk outside with your coffee, eat breakfast by a window, whatever sticks. Phase shifts happen within days.

If you suspect caffeine is affecting your sleep: Run an experiment. Move your cutoff to noon for two weeks, or cut it entirely for ten days if you're feeling ambitious. Track your sleep subjectively or with a wearable, and see what changes. Most people are surprised.

If alcohol is a regular part of your routine: You don't have to quit, but get honest about the tradeoff. Check your HRV the morning after drinking and compare it to your baseline. Once you see the data, you can make informed decisions about when it's worth it, and when it's not.

The non-negotiable minimum: Seven to eight hours of sleep opportunity (time in bed, not just time asleep). Morning light daily. Some form of movement most days—ideally including Zone 2 cardio at least three times a week.

What's Next

Exercise and sleep are the load-bearing walls. Get these right, and everything else in the stack works better. Get them wrong, and you're constantly compensating.

But there's more to the environment than your bedroom temperature.

Next up: Temperature and Environment—cold exposure, sauna, CO2 and air quality, and why the people around you are part of your "environment" too.

[And one closing note: the interventions in this post work for most people—but "most people" isn't the same as you, specifically. At A3, we combine biomarker data, AI analysis, and ongoing coaching to help clients figure out exactly which protocols will move the needle for their particular physiology and goals. If you want help building a personalized stack rather than experimenting on your own, we're here to help.]