# Twenty-eight Navy divers descended to a simulated 220 feet and experience showed up not in reported fear but in cortisol, heart-rate variability and steadier recovery during decompression

> Twenty-eight Navy divers entered a simulated deep dive to 220 feet and the clearest split between newcomers and veterans did not come from what they said they felt. It came from what their bodies did under pressure. The study found that experienced...

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Byline: ARGO.net Editorial Team
Published: 2026-08-07T10:55:03+00:00
Categories: Explainer, Humans

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Twenty-eight Navy divers entered a simulated deep dive to 220 feet and the clearest split between newcomers and veterans did not come from what they said they felt. It came from what their bodies did under pressure. The study found that **experienced Navy divers** and novice divers reported no major difference in self-rated stress, anxiety, or fear, yet the veterans showed stronger signs of autonomic control and a cleaner biochemical recovery after the dive.

Researchers from **National Defense Medical Center**, **Tri-Service General Hospital** and collaborating Taiwanese military diving units described the work in [Frontiers in Physiology](https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2025.1642779/full). They tracked **salivary cortisol**, salivary amylase and **heart rate variability** before the dive, at the bottom stage, during decompression and after the simulation ended. Their central result was unusually practical: the mind's verbal report and the body's stress signature did not fully match.

That mismatch matters for military diving because deep dives combine task pressure, unusual breathing conditions and a strict ascent profile where mistakes can become dangerous very quickly. A diver who sounds calm may still be carrying a high hidden load. The study argues that experience can build a more resilient pattern of physiological adaptation, especially during the demanding transition out of depth.

## Hidden stress showed up in the body

The paper divided the 28 participants into 15 experienced divers and 13 novice divers, then compared psychological questionnaires with physiological markers. On the questionnaire side, the groups looked surprisingly similar. After the researchers adjusted for age and perceived stress, no significant difference appeared in self-reported psychological stress between the more experienced divers and the novices.

Physiology told a different story. The experienced group showed greater post-dive reductions in cortisol and amylase, two saliva-based markers often used to follow stress-related activation. They also showed higher parasympathetic activity and greater complexity in heart rhythm patterns, especially during the **decompression** phase. In simple terms, the veterans did not merely endure the dive. Their nervous systems appeared to regulate it more smoothly.

That finding gives the article its strongest human angle. Stress adaptation is often discussed as if it were obvious on the surface, but the study suggests that adaptation can stay largely invisible to conversation and self-description. A diver may believe the situation feels manageable, yet the **autonomic nervous system** can still reveal who has practiced these conditions enough to recover more efficiently.

## Why decompression exposed the biggest gap

The largest contrast between the groups emerged during decompression, the staged return toward normal pressure. This phase places special demands on the body because the diver is no longer simply enduring depth. The body is also adjusting to a changing environment while managing gas elimination and maintaining control through a constrained ascent process. The study found that novice divers showed weaker autonomic adaptation here than the experienced group.

At that point in the dive, the researchers saw higher values in several heart rate variability measures among the experienced divers, including rMSSD, HF and SDNN, along with stronger performance in **sample entropy**, a non-linear measure that reflects the complexity of beat-to-beat variation. The paper interprets that richer variability as a sign of more flexible regulation rather than a rigid, overstrained response.

Earlier diving studies point in the same direction. A [2021 Frontiers study on heart rate variability during a standard dive](https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2021.635132/full) also reported that diving can push heart and breathing regulation into a different autonomic pattern at depth. The new Navy diver paper adds a sharper conclusion: the difficult part is not only the bottom stage, but the body's ability to reorganize itself while rising out of that environment.

## Low fear scores do not mean low load

One of the most interesting lessons in the paper is psychological. Novice divers did not report dramatically higher fear, even though their physiological signals suggested a heavier burden. That gap can happen for several reasons. People may underread their own state, military culture may reward calm reporting and some forms of stress are easier to detect in pulse and hormones than in a short questionnaire answered around a demanding exercise.

The authors raise that possibility directly in their conclusion, suggesting that the missing difference in self-report may reflect the limits of subjective reporting rather than a genuine absence of psychological strain. For training officers, that is more than a technical footnote. It means a diver's spoken confidence should be treated as one layer of evidence, not the whole picture.

Related work has shown that diving conditions can reshape autonomic behavior even when external signs remain subtle. A [2019 study of Finnish Navy divers in Arctic water](https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2019.01600/full) found measurable heart rate variability responses in extreme cold, emphasizing how much the underwater environment can influence the body before a diver says anything is wrong. The new 220-foot simulation extends that lesson into the realm of experience and hidden stress.

## Experience may train the nervous system

The study's broader claim is that repeated exposure to deep diving may foster a more resilient regulatory pattern. Across the simulated dive stages, experienced divers tended to show lower mean heart rates and stronger parasympathetic measures than novices. The pattern fits an appealing idea in performance science: under repeated, well-managed stress, the body can learn to switch out of high alert more effectively.

That does not mean experience makes deep diving easy or safe by itself. It means training may help the body conserve adaptability when pressure changes, breathing patterns and mental workload all converge. The paper points to better post-dive recovery in the veterans, which suggests a form of **trained autonomic plasticity**. Instead of lingering in a prolonged stress state, the experienced group appeared more able to rebalance after the challenge eased.

Monitoring technology could make that adaptation easier to track in real time. A [2024 review on physiological monitoring to prevent diving disorders](https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2024.1517361/full) argued that divers may benefit from broader sensing of the signals that precede trouble, while a [JMIR scoping review of wearable devices in diving](https://mhealth.jmir.org/2022/9/e35727) found growing interest in underwater wearables but also noted gaps, especially around practical sensing during real operations. The new Navy diver study gives those monitoring efforts a sharper target: hidden stress adaptation during decompression and recovery.

## Useful findings still come with limits

The paper is informative, but it is not the final word on diver psychology. The sample was small, as many military physiology studies are and only one woman was included, which limits generalization. The researchers also note several factors they could not fully control, including sleep quality, recent physical or mental workload and other personal traits that can influence cortisol and amylase levels.

Movement during underwater measurement is another limitation. Even though participants were asked to stay still during recording windows, diving is not a resting laboratory setting. The team also had to estimate respiratory rate from beat-to-beat intervals instead of measuring breathing directly during the dive. Because breathing strongly affects heart rate variability, future studies with direct respiratory monitoring could sharpen the picture.

Even with those caveats, the result is strong enough to guide practice. The safest reading is modest and useful: **deep diving experience** appears to improve physiological resilience, especially when the body is coming off pressure and self-reported calm does not fully capture that adaptation. For instructors and medical planners, the implication is clear. Training quality and real-time monitoring may reveal hidden overload sooner than interviews alone, which could make high-pressure operations safer for divers who are still learning to manage the strain under demanding conditions.