When the Body's Clock Breaks: How Burnout Rewrites Athletes' Physiological Rhythms

There's a particular kind of physiological unraveling that happens when athletes push past sustainable limits for extended periods. The body doesn't just get tired—it starts operating under fundamentally altered rules, its regulatory systems losing the rhythmic patterns that normally govern stress response, recovery, and adaptation. We can measure this breakdown in heart rate variability that becomes erratic and suppressed, in cortisol rhythms that flatten and invert, in muscles that remain chronically fatigued despite rest. These aren't just symptoms of being overworked. They're evidence that burnout has rewritten the athlete's physiology at a systemic level.

The research on physiological markers of athlete burnout reveals patterns so consistent they should serve as alarm bells for anyone involved in elite sport development. Heart rate variability—the beat-to-beat variation in heart rhythm that reflects autonomic nervous system function—shows particularly reliable changes in burned-out athletes (Moore et al., 2024; Mishica et al., 2021; Iizuka et al., 2020; Schmitt et al., 2013; DeBlauw et al., 2023; Olmos et al., 2024). Specifically, athletes experiencing burnout and chronic fatigue demonstrate reduced vagally-mediated HRV at rest, reflecting suppressed parasympathetic activity and autonomic imbalance that prevents adequate recovery.

This HRV reduction isn't subtle or contested—it appears consistently across studies and populations. Athletes in burnout states show decreased markers like RMSSD, SDNN, and high-frequency power, all indicating that the parasympathetic nervous system—the "rest and digest" system responsible for recovery—has been chronically suppressed (Mishica et al., 2021; Iizuka et al., 2020; DeBlauw et al., 2023; Olmos et al., 2024). The autonomic nervous system gets stuck in a state where stress response dominates and recovery mechanisms can't adequately activate even during periods meant for rest.

What makes HRV particularly valuable as a burnout marker is how it captures something athletes themselves often can't articulate or even recognize. The burned-out athlete might not consciously register that their recovery capacity is impaired, might interpret persistent fatigue as lack of fitness requiring more training rather than as physiological breakdown requiring rest. But HRV measurement reveals the underlying autonomic dysfunction that prevents recovery regardless of training volume or sleep quantity. The numbers show what subjective assessment misses: that the body has lost capacity to downregulate stress systems effectively.

The increased intra-individual variability in HRV parameters adds another dimension to this picture (Schmitt et al., 2013). Not only is HRV generally lower in burned-out athletes, but it becomes unstable—fluctuating more from day to day in ways that indicate dysregulated autonomic control rather than healthy adaptive variation. This instability suggests the regulatory systems themselves are compromised, unable to maintain consistent functioning even under relatively stable external conditions. The athlete's physiological state becomes unpredictable, which makes training response equally unpredictable and increases vulnerability to further breakdown.

Cortisol rhythms provide another window into burnout's physiological signature, one that reveals how chronic stress fundamentally alters the hypothalamic-pituitary-adrenal axis that governs stress hormone production. Healthy cortisol patterns follow a strong circadian rhythm: levels peak in early morning to support awakening and energy mobilization, then decline throughout the day to reach minimal levels at night (Hayes et al., 2010; Nobari et al., 2023; Augsburger et al., 2025). This rhythm isn't just about timing—it's essential for balancing energy availability with recovery needs, for allowing the body to alternate between active stress response and restorative downregulation.

Burnout disrupts this rhythm in characteristic ways. Athletes experiencing burnout show elevated salivary cortisol, particularly at bedtime, when levels should be at their lowest (Moore et al., 2024; Monfared et al., 2020; Mishica et al., 2021; Botelho et al., 2020; Soler-López et al., 2024). The evening elevation means the body never fully enters the hormonal state necessary for deep recovery—stress hormones remain elevated when they should be suppressed, preventing the restorative processes that normally occur during sleep. The athlete might be physically lying in bed, but physiologically they're still in a state of stress arousal that prevents genuine rest.

The flattening of the diurnal cortisol rhythm represents equally troubling dysfunction. In burned-out athletes, the difference between morning peak and evening trough diminishes—morning levels fail to rise adequately, evening levels fail to fall sufficiently, creating a flattened pattern that reflects chronic HPA axis activation and impaired stress recovery (Moore et al., 2024; Sánchez et al., 2021; Adam et al., 2017). This flattening isn't just a shift in timing but a loss of rhythmic regulation itself. The body has been operating under chronic stress for so long that its capacity to modulate stress hormone production according to circadian and contextual cues has deteriorated.

The cortisol awakening response—the rapid increase in cortisol that should occur within thirty minutes of waking—shows variable patterns in burnout that likely reflect different stages or types of HPA axis dysfunction (Kadooka et al., 2024; Anderson et al., 2021; Tsunekawa et al., 2023). Some burned-out athletes display heightened CAR, suggesting hyperresponsive stress systems. Others show blunted or absent CAR, indicating that the HPA axis has become exhausted and can no longer mount normal arousal responses (Ushiki et al., 2020; Tsunekawa et al., 2023). Both patterns are problematic—hyperresponsivity means the body overreacts to normal waking, exhaustion means it can't generate the cortisol surge necessary for optimal morning alertness and function.

The progressive nature of cortisol disruption deserves attention. Across a training week or competitive season, athletes developing burnout show increasingly dysregulated patterns: higher evening cortisol, reduced morning peaks, eventual reversal of the normal circadian pattern (Sánchez et al., 2021). This suggests burnout isn't a sudden switch but a gradual degradation of physiological regulation, where the HPA axis's capacity to maintain healthy rhythms erodes under sustained excessive demand. By the time the pattern has fully flattened or inverted, the athlete is in advanced burnout requiring substantial recovery time to restore normal function.

The relationship between cortisol rhythm and mood disturbance provides additional evidence of burnout's psychological-physiological integration. Higher cortisol levels correlate with negative mood states, increased tension, and confusion—the psychological symptoms of burnout track with the hormonal dysregulation (Botelho et al., 2020). This isn't coincidental. The same HPA axis disruption that elevates evening cortisol also affects neurotransmitter systems involved in mood regulation. The athlete isn't just stressed about training—their disrupted stress hormone patterns are literally generating the mood disturbance they're experiencing.

Muscle fatigue and damage markers complete the picture of burnout's physiological signature. Athletes in burnout states show elevated creatine kinase and other markers of muscle damage, increased soreness, and reduced muscle recovery capacity (Soler-López et al., 2024). This persistent muscle dysfunction isn't adequately explained by training volume alone—healthy athletes training at comparable volumes show better recovery and lower damage markers. The difference is that burned-out athletes have lost the hormonal environment necessary for effective muscle repair and adaptation.

The mechanism here likely involves the disrupted cortisol patterns already discussed. Chronically elevated cortisol creates a catabolic environment that promotes muscle breakdown while impairing the anabolic processes necessary for recovery and growth (Kraemer et al., 2020; Nobari et al., 2023). The flattened circadian rhythm means the athlete never experiences the low-cortisol window that would normally occur in evening and overnight, when muscle repair processes are most active (Hayes et al., 2010; Teo et al., 2011; Bird & Tarpenning, 2004). Their muscles are trying to recover while bathed in stress hormones that actively prevent recovery, which explains why rest doesn't resolve the fatigue and damage accumulates despite reduced training.

The normal relationship between training timing and hormonal response gets disrupted in burnout as well. Healthy athletes show predictable circadian variations in performance and recovery—strength and power peak in late afternoon when cortisol is lower and body temperature higher, evening training produces faster recovery than morning training, the testosterone-to-cortisol ratio favors muscle growth more during evening hours (Hayes et al., 2010; Nobari et al., 2023; Teo et al., 2011; Bird & Tarpenning, 2004; Erdemir et al., 2013). But burned-out athletes with flattened cortisol rhythms lose these circadian performance advantages. Their hormonal environment remains relatively catabolic throughout the day, preventing the afternoon/evening optimization that healthy circadian rhythms provide.

What makes these physiological signatures particularly valuable is their objectivity. Athletes experiencing burnout often struggle to accurately assess their own state—they may deny deteriorating performance, attribute symptoms to other causes, or feel pressured to maintain training despite clear distress. But HRV measurement, cortisol sampling, and muscle damage markers provide data independent of the athlete's subjective interpretation (Moore et al., 2024). The numbers can reveal burnout that athletes are unable or unwilling to acknowledge themselves, allowing for earlier intervention before complete breakdown occurs.

However, implementing systematic monitoring of these physiological markers requires athletic organizations that value long-term athlete health over short-term performance maximization. Regular HRV assessment costs time and resources. Cortisol sampling involves coordinating collection at specific times across multiple days. Interpreting the data requires expertise in exercise physiology and endocrinology. Many programs lack the infrastructure or commitment to conduct this monitoring, and some actively resist it because they don't want objective evidence that might contradict their training philosophies or require reducing training loads.

There's also complexity around what constitutes abnormal values and when intervention is warranted. Individual athletes have different baseline HRV and cortisol patterns, so changes within an athlete over time often matter more than absolute values compared to population norms. The variability in cortisol awakening response patterns means burnout doesn't always look identical across athletes. Some physiological changes might represent acute fatigue that resolves with brief recovery rather than chronic burnout requiring extended intervention. Distinguishing between these requires longitudinal monitoring and sophisticated interpretation that goes beyond simple cutoff values.

The research makes clear that burnout produces measurable, consistent physiological signatures—reduced and unstable HRV, elevated and dysregulated cortisol rhythms, persistent muscle fatigue and damage (Moore et al., 2024; Mishica et al., 2021; Monfared et al., 2020; Soler-López et al., 2024). These markers can complement self-report tools to enable earlier detection and more objective assessment of athlete state. They reveal that burnout isn't primarily psychological weakness or motivational failure but systemic physiological breakdown affecting autonomic regulation, stress hormone production, and muscle recovery capacity.

Whether elite sport will actually implement monitoring systems that could detect these patterns early, and whether organizations will respond appropriately when monitoring reveals burnout, remains uncertain. The tools exist, the markers are well-characterized, the interpretation frameworks are available. What's less clear is whether the institutional will exists to prioritize prevention of physiological breakdown over extraction of maximum performance regardless of cost to athlete health. The physiological signatures of burnout tell us when bodies are breaking down. Whether we're willing to listen to what those signatures are saying about how we structure athletic development is a different question entirely.

References

Adam, E. K., Quinn, M. E., Tavernier, R., McQuillan, M. T., Dahlke, K. A., & Gilbert, K. E. (2017). Diurnal cortisol slopes and mental and physical health outcomes: A systematic review and meta-analysis. Psychoneuroendocrinology, 83, 25–41. https://doi.org/10.1016/j.psyneuen.2017.05.018

Anderson, T., Wideman, L., Cadegiani, F. A., & Kater, C. E. (2021). Effects of overtraining status on the cortisol awakening response-endocrine and metabolic responses on overtraining syndrome (EROS-CAR). International Journal of Sports Physiology and Performance, 16(10), 1392–1400. https://doi.org/10.1123/ijspp.2020-0205

Augsburger, G., Sobolewski, E. J., Escalante, G., & Graybeal, A. J. (2025). Circadian regulation for optimizing sport and exercise performance. Clocks & Sleep, 7(1), 133–152. https://doi.org/10.3390/clockssleep7020018

Bird, S. P., & Tarpenning, K. M. (2004). Influence of circadian time structure on acute hormonal responses to a single bout of heavy-resistance exercise in weight-trained men. Chronobiology International, 21(1), 131–146. https://doi.org/10.1081/cbi-120027987

Botelho, R. P., Abad, C. C. C., Spadari, R. C., Winckler, C., Garcia, M. C., & Guerra, R. L. P. (2020). Psychophysiological stress markers during preseason among elite female soccer players. Journal of Strength and Conditioning Research, 36(6), 1648–1654. https://doi.org/10.1519/jsc.0000000000003702

DeBlauw, J. A., Stein, J. C., Blackman, C. G., Haas, M. R., Makle, S. M., Echevarria, I. S., Edmonds, R., & Ives, S. J. (2023). Heart rate variability of elite female rowers in preparation for and during the national selection regattas: A pilot study on the relation to on water performance. Frontiers in Sports and Active Living, 5, Article 1245788. https://doi.org/10.3389/fspor.2023.1245788

Erdemir, İ., Kizilet, A., & Bozdoğan, T. U. (2013). Effects of exercise on circadian rhythms of cortisol. International Journal of Sports Science, 3(2), 68–73.

Hayes, L. D., Bickerstaff, G. F., & Baker, J. S. (2010). Interactions of cortisol, testosterone, and resistance training: Influence of circadian rhythms. Chronobiology International, 27(4), 675–705. https://doi.org/10.3109/07420521003778773

Iizuka, T., Ohiwa, N., Atomi, T., Shimizu, M., & Atomi, Y. (2020). Morning heart rate variability as an indication of fatigue status in badminton players during a training camp. Sports, 8(11), Article 147. https://doi.org/10.3390/sports8110147

Kadooka, S., Ogasawara, Y., Sugo, T., & Tsuchiya, H. (2024). Effects of burnout tendencies on cortisol awakening response in athletes. International Journal of Sport and Health Science, 22, 51–61. https://doi.org/10.5432/ijshs.202214

Kraemer, W. J., Ratamess, N. A., Hymer, W. C., Nindl, B. C., & Fragala, M. S. (2020). Growth hormone(s), testosterone, insulin-like growth factors, and cortisol: Roles and integration for cellular development and growth with exercise. Frontiers in Endocrinology, 11, Article 33. https://doi.org/10.3389/fendo.2020.00033

Mishica, C., Kyröläinen, H., Hynynen, E., Nummela, A., Holmberg, H. C., & Linnamo, V. (2021). Relationships between heart rate variability, sleep duration, cortisol and physical training in young athletes. Journal of Sports Science & Medicine, 20(4), 778–788. https://doi.org/10.52082/jssm.2021.778

Monfared, S., Lebeau, J. C., Mason, J., Cho, S., Basevitch, I., Perry, I., Baur, D. A., & Tenenbaum, G. (2020). A bio-physio-psychological investigation of athletes' burnout. Research Quarterly for Exercise and Sport, 92(2), 189–198. https://doi.org/10.1080/02701367.2020.1715911

Moore, L. J., Isoard-Gautheur, S., & Gustafsson, H. (2024). Psychophysiological markers of athlete burnout: A call to arms. International Journal of Sports Medicine, 45(13), 955–964. https://doi.org/10.1055/a-2433-3930

Nobari, H., Azarian, S., Saedmocheshi, S., Valdés-Badilla, P., & Calvo, T. G. (2023). Narrative review: The role of circadian rhythm on sports performance, hormonal regulation, immune system function, and injury prevention in athletes. Heliyon, 9(9), Article e19636. https://doi.org/10.1016/j.heliyon.2023.e19636

Olmos, M., Capdevila, L., & Caparrós, T. (2024). Heart rate variability in elite team sports: A systematic review. Open Access Journal of Disease and Global Health, 2(3), 1–15. https://doi.org/10.33140/oajdgh.02.03.01

Sánchez, I., De La Rubia Ortí, J. E., Platero, J. L., Mariscal, G., & Barrios, C. (2021). Modification of diurnal cortisol secretion in women's professional basketball: A pilot study. International Journal of Environmental Research and Public Health, 18(16), Article 8961. https://doi.org/10.3390/ijerph18168961

Schmitt, L., Regnard, J., Desmarets, M., Mauny, F., Mourot, L., Fouillot, J. P., Coulmy, N., & Millet, G. P. (2013). Fatigue shifts and scatters heart rate variability in elite endurance athletes. PLOS ONE, 8(8), Article e71588. https://doi.org/10.1371/journal.pone.0071588

Soler-López, A., Moreno-Villanueva, A., Gómez-Carmona, C. D., & Pino-Ortega, J. (2024). The role of biomarkers in monitoring chronic fatigue among male professional team athletes: A systematic review. Sensors, 24(21), Article 6862. https://doi.org/10.3390/s24216862

Teo, W., Newton, M. J., & McGuigan, M. R. (2011). Circadian rhythms in exercise performance: Implications for hormonal and muscular adaptation. Journal of Sports Science & Medicine, 10(4), 600–606.

Tsunekawa, K., Shoho, Y., Ushiki, K., Yanagawa, Y., Matsumoto, R., Shimoda, N., Aoki, T., Yoshida, A., Nakajima, K., Kimura, T., & Murakami, M. (2023). Assessment of exercise-induced stress via automated measurement of salivary cortisol concentrations and the testosterone-to-cortisol ratio: A preliminary study. Scientific Reports, 13(1), Article 13821. https://doi.org/10.1038/s41598-023-41620-5

Ushiki, K., Tsunekawa, K., Shoho, Y., Martha, L. S., Ishigaki, H., Matsumoto, R., Yanagawa, Y., Nakazawa, A., Yoshida, A., Nakajima, K., Araki, O., Kimura, T., & Murakami, M. (2020). Assessment of exercise-induced stress by automated measurement of salivary cortisol concentrations within the circadian rhythm in Japanese female long-distance runners. Sports Medicine - Open, 6(1), Article 35. https://doi.org/10.1186/s40798-020-00269-4