Katie Wood
Freediver, Writer, Explorer
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Katie Wood
Freediver, Writer, Explorer
For most freedivers, the descent into the deep is an act of surrender. One full breath, a moment of stillness, and gravity takes over. The noise of the surface fades, and the body begins to transform. Heart rate slows, blood vessels narrow, and oxygen flow is redirected toward the organs that matter most. This is the mammalian dive reflex, one of the body’s oldest and most elegant survival mechanisms. It is what allows us to glide through the depths on a single breath. Yet even this ancient reflex can be weakened by a force most of us carry unknowingly every day: stress.
Modern life has altered the chemistry of our bodies. We live surrounded by invisible alarms that keep our nervous system in a constant state of readiness. The body’s main stress hormone, cortisol, becomes both a protector and a saboteur. When balanced, it keeps us alert and capable. When elevated for too long, it disrupts sleep, weakens recovery, and interferes with the physiological calm that freediving requires. Understanding cortisol is therefore not only about managing stress but about preserving the very essence of the dive reflex itself.
To understand how stress affects freediving, we must begin with its biology. When the brain perceives a threat, whether real or imagined, it activates the hypothalamic pituitary adrenal axis. This system releases a surge of cortisol from the adrenal glands. Cortisol’s role is to prepare the body for action. It raises blood sugar, increases alertness, and mobilizes energy reserves. In small, controlled bursts, it is essential. It sharpens focus and enhances reaction time.
However, when stress becomes continuous, the balance between the sympathetic and parasympathetic systems begins to erode. The sympathetic system governs the fight or flight response, increasing heart rate and blood pressure. The parasympathetic system governs recovery, digestion, and relaxation. Freediving depends entirely on the parasympathetic side. It is what allows the heart to slow down and oxygen to be conserved. Chronic stress keeps the sympathetic system dominant, leaving the parasympathetic response underactive. The result is a nervous system that struggles to relax even when you try to.
For a freediver, this imbalance is costly. Elevated cortisol levels mean higher resting heart rates, shorter breath holds, and slower recovery between dives. The body’s ability to switch into energy saving mode diminishes. Even before submersion, subtle tension can be felt in the chest or diaphragm. The relaxation that once came naturally now requires conscious effort.
The mammalian dive reflex is a sophisticated orchestration of heart, blood, and lungs. It lowers cardiac output, redirects blood flow toward the core, and triggers the spleen to release additional red blood cells. Each part of this process depends on a clear signal from the parasympathetic nervous system. When cortisol remains elevated for long periods, this signaling weakens.
High cortisol interferes with the vagus nerve, which is responsible for the heart’s slowing response during apnea. Instead of smoothly shifting into bradycardia, the heart rate may stay slightly elevated, wasting oxygen. Peripheral vasoconstriction, which helps conserve blood oxygen for vital organs, may become less effective. These subtle disruptions shorten safe dive times and increase recovery needs.
Cortisol also influences how oxygen is delivered at the cellular level. It alters hemoglobin’s affinity for oxygen, slightly changing the way it binds and releases it. In a chronic stress state, the tissues may receive less oxygen than they should, even when supply appears normal. Over time, this reduces both performance and endurance.
The hormonal interplay does not stop there. Cortisol suppresses growth hormone, testosterone, and thyroid activity. These hormones are crucial for muscle repair and metabolic stability. A diver who trains frequently under stress may feel constantly fatigued without understanding why. Muscles recover slower, and the immune system weakens, increasing the risk of illness between training cycles.
The paradox is that freedivers often live between two worlds. In one world, there is stillness, depth, and control. In the other, there is traffic, screens, deadlines, and constant stimulation. The nervous system rarely gets a chance to reset. Cortisol is designed for brief, acute challenges, not the relentless pace of modern life. The more it stays active, the more it reshapes the body’s baseline state.
A chronically stressed diver may begin to notice that even on rest days, relaxation feels elusive. The breathing patterns used for preparation may not slow the heart as effectively as before. Static tables feel harder. Equalization requires more focus. The cause is not lack of training but biochemical resistance. The body no longer interprets calm as safety because it has forgotten how to switch off.
Sleep becomes one of the first casualties. Cortisol naturally follows a rhythm, rising in the morning and falling at night. When this rhythm is disrupted, sleep quality declines, and the next day begins with cortisol already elevated. Poor sleep leads to lower heart rate variability, higher inflammation, and reduced tissue recovery. Over time, this creates a feedback loop: poor sleep raises cortisol, high cortisol impairs sleep, and performance gradually declines.
Freediving is a discipline of efficiency. Every molecule of oxygen counts. Hormones play a silent yet powerful role in determining how effectively that oxygen is used. Cortisol increases metabolism, which is beneficial in short bursts but counterproductive for long breath holds. Elevated cortisol levels accelerate energy consumption even during rest. This means oxygen stores deplete faster, and contractions arrive sooner.
During deep dives, the body depends on the ability to reduce heart rate and redirect circulation. This process is partly mechanical and partly hormonal. Cortisol interferes with both aspects. It reduces the responsiveness of baroreceptors that regulate blood pressure, making the adjustment to pressure changes slower. It also blunts the spleen’s ability to release extra red blood cells, reducing oxygen availability during long dives.
On the metabolic side, cortisol encourages the breakdown of muscle protein for glucose. Over time, this catabolic effect can reduce strength and muscular endurance. For a freediver, whose performance relies on efficient and powerful finning, this degradation can subtly impair propulsion and increase energy cost per movement.
Freedivers are often highly disciplined athletes. They track performance, refine technique, and repeat sessions for precision. But overtraining can mimic the effects of chronic stress. When training intensity exceeds recovery capacity, cortisol remains high for prolonged periods. Eventually, the adrenal glands begin to produce less, leading to hormonal exhaustion. The symptoms are familiar: low energy, irritability, poor sleep, and slower recovery.
What begins as dedication can become depletion. The body that once adapted through stress now resists it. Even moderate dives may feel demanding. The lesson here is not to avoid hard training but to balance it with genuine recovery. Stress and adaptation are two sides of the same process. Without recovery, there is no growth.
Recovery is not passive. It is an active process of restoring balance within the endocrine system. The most powerful tool for regulating cortisol is sleep. During deep sleep, cortisol levels drop while growth hormone rises. This hormonal exchange repairs tissues, strengthens the nervous system, and resets the stress axis. Consistent, high quality sleep has more influence on freediving performance than many realize.
Nutrition also plays a critical role. Diets chronically low in carbohydrates or calories can raise cortisol levels, as the body interprets low energy availability as a form of stress. A balanced diet that supports glycogen replenishment helps stabilize both energy and hormonal responses. Adequate hydration further aids in cortisol regulation, as dehydration amplifies the stress response and increases perceived exertion.
Mental recovery is equally important. Techniques such as meditation, visualization, and slow breathing reduce sympathetic activation and lower cortisol. For freedivers, these are not abstract practices but practical skills. Controlled breathing before a dive activates the vagus nerve and primes the body for the shift into the dive reflex. Over time, such routines strengthen the body’s ability to transition from stress to calm on demand.
The dive reflex is a living feedback system. It mirrors the diver’s internal state. When you are rested and balanced, the heart slows easily, contractions come late, and the dive feels smooth. When you are stressed or overworked, the reflex becomes sluggish. Each dive can therefore act as a diagnostic tool for your overall health.
If heart rate does not drop as much as usual, if recovery feels slower, or if breath holds feel shallow, the cause may not be lack of fitness but hormonal imbalance. Recognizing these signals early allows you to adjust training, increase recovery days, or address lifestyle stressors before they accumulate.
Freedivers who approach the sport as a long term practice often learn that sustainable performance is not about pushing limits constantly but about managing energy intelligently. Cortisol management becomes a cornerstone of that sustainability.
Cortisol cannot be eliminated, nor should it be. The goal is to maintain its rhythm. Exposure to natural light in the morning helps regulate its daily cycle. Regular meals prevent energy dips that trigger stress responses. Moderate physical activity on rest days promotes recovery without overstimulation.
Social connection also helps regulate cortisol. Sharing time with friends or training partners releases oxytocin, a hormone that counteracts cortisol’s effects. Even laughter has been shown to lower cortisol levels. The lesson is simple: recovery does not always require solitude. Sometimes, it thrives on connection.
Breathing remains the most immediate tool for control. Slow, diaphragmatic breathing signals safety to the nervous system. By extending the exhalation, you activate parasympathetic pathways that calm the heart. This technique can be used on land, before sleep, or as part of dive preparation. Over time, it becomes a natural switch from alertness to tranquility.
Freediving is not only about physiology. It is about self observation. The ocean reflects the state you bring into it. When you carry tension, it becomes visible in every movement. When you are calm, the water amplifies that calmness. In this sense, cortisol becomes more than a hormone; it becomes a messenger. Elevated cortisol before a dive may be a sign to rest, to breathe longer, to slow down. Low motivation after intense training might signal that recovery has not yet caught up.
By paying attention to these signals, freedivers can turn cortisol from an adversary into a teacher. It tells you when to push and when to pause, when to go deeper and when to stay still. This awareness transforms freediving from a sport into a form of self regulation, where every descent becomes both physical and emotional exploration.
Sustaining high level performance in freediving requires more than strong lungs or perfect technique. It requires hormonal resilience. Cortisol management is central to that resilience. Depth records and personal bests matter less than longevity, the ability to keep diving year after year without burnout or injury.
By respecting the body’s natural rhythms, divers cultivate consistency. They learn that calm is not a passive state but a trained response. The more consistently you balance stress and recovery, the deeper and safer your dives become. The dive reflex, once dulled by stress, reawakens in full strength, and the ocean once again feels like home.
Freediving teaches that control begins with surrender. The same principle applies to stress. You cannot fight cortisol any more than you can fight the ocean. You can only learn to move with it, to understand its signals, and to restore balance through awareness. When the heart slows and the body softens into the rhythm of the deep, the chemistry of stress no longer dominates. What remains is harmony.
In that quiet space below the surface, the body remembers its original intelligence. The reflexes align, the oxygen lasts, and time stretches. You are no longer swimming against yourself. You are part of the water, part of the stillness, part of a world where stress dissolves and only presence remains. This is what freediving, at its deepest level, really teaches: how to find calm not by escaping stress, but by mastering it.