Narrative review: The role of circadian rhythm on sports performance, hormonal regulation, immune system function, and injury prevention in athletes

The review addresses how the circadian rhythm (CR), governed by the suprachiasmatic nucleus (SCN), shapes daily physiological processes relevant to athletic performance, including sleep-wake cycles, thermoregulation, hormonal secretion, and neuromuscular function. It explores why many performance metrics peak in the afternoon alongside core body temperature and how diurnal hormonal patterns (e.g., morning peaks of cortisol and testosterone) influence performance. The purpose is to synthesize evidence on CR effects on sports performance, hormonal regulation, immune function, fatigue/overtraining, nutrition, sex differences, and injury prevention, underscoring the importance of aligning training, recovery, and sleep with individual chronotype and time-of-day effects.

The narrative synthesis compiles evidence that CR modulates motor and psychomotor skills, perceptual/cognitive functions, and physical outputs. Core body temperature shows a nadir pre-dawn and peaks late afternoon, paralleling improved coordination, reaction time, strength, anaerobic power, and flexibility. Anaerobic performance often peaks later in the day, with mixed findings for aerobic capacity, while habitual training time can shift performance peaks. Diurnal hormone rhythms (melatonin, cortisol, growth hormone, leptin, ghrelin, thyroid-stimulating hormone) and sleep stages interact, influencing recovery, metabolism, and performance. Immune function exhibits circadian oscillations in leukocyte trafficking, cytokines, and inflammatory responses, with central and peripheral clocks (e.g., immune cells) coordinating timing. Fatigue-related biomarkers (lactate, CK, LDH, homocysteine), antioxidant status, and oxidative stress show time-of-day variation, often favoring higher anaerobic metabolism and muscle damage markers in the evening. Sleep deficiency contributes to non-functional overreaching/overtraining risks, impaired immune function, hormonal imbalances, and increased injury, particularly in adolescents. Exposure to short-wavelength evening light suppresses melatonin and alters sleep architecture, potentially impairing next-morning alertness. Nutritional chrono-strategies (carbohydrate timing, tryptophan-rich proteins, melatonin/magnesium/zinc) can modulate sleep and CR. Sex differences exist across development, with females generally reporting more insomnia yet obtaining longer objective sleep and showing differing homeostatic responses to sleep loss.

Narrative review guided by PRISMA principles. Databases: PubMed, Scopus, and Web of Science (Core Collection). Search terms included "circadian rhythm", "sports performance", "hormonal regulation", "immune system", and "injury prevention", clustered under a PICO framework (population, phenomenon of interest, context) to ensure specificity. Inclusion criteria: English-language, peer-reviewed studies published up to July 2023, examining CR in relation to sports performance, hormonal regulation, immune function, and injury prevention in athletes. Studies were reviewed and synthesized narratively, describing mechanisms, links between CR and endocrine homeostasis, sex differences, and immune coordination. The approach combined systematic elements to improve rigor while retaining narrative synthesis.

- CR influences nearly all physiological and biochemical processes; the SCN aligns internal rhythms with the light-dark cycle, affecting sleep, thermoregulation, hormones, and behavior. - Performance of many metrics peaks in the late afternoon/evening, coincident with peak core body temperature; body temperature may be ~0.9°C higher in the afternoon, facilitating muscle function, carbohydrate utilization, and contractile properties. - Time-of-day of exercise affects outcomes: anaerobic power tends to peak in the late afternoon/evening; habitual training time can shift acrophases; technical skill tasks may peak earlier than maximal power. - Diurnal variations in biomarkers: higher evening lactate responses, homocysteine, CK/LDH, and muscle damage markers; lower morning antioxidant defenses vs evening patterns vary across studies; diurnal redox and enzyme activities relate to performance and fatigue. - Hormonal diurnal patterns: cortisol and testosterone peak in the morning and decline across the day; melatonin peaks at biological night; GH pulses are linked to SWS; leptin/ghrelin show meal- and sleep-related rhythms; these modulate recovery, metabolism, and performance. - Immune system exhibits circadian control over leukocyte trafficking, cytokine expression, and inflammatory responses; peripheral clocks in immune cells integrate with central SCN signals. - Sleep is central to recovery and CR stability; insufficient sleep relates to elevated catabolic milieu, reduced anabolic signaling, impaired immune responses, and higher injury risk. Adolescents sleeping <8 h show greater injury risk (e.g., OR ~2.1; 95% CI 1.2–3.9), while >8 h lowers risk (OR 0.39; 95% CI 0.17–0.96). - Light exposure effects: evening short-wavelength (∼460 nm) light suppresses melatonin, reduces SWS/SWA in the first sleep cycle, and may impair next-morning alertness. - Nutritional chrono-strategies: high-GI carbohydrate in the evening can reduce sleep onset latency; tryptophan-rich proteins and melatonin supplementation can improve sleep efficiency and quality. A melatonin–magnesium–zinc supplement improved subjective sleep quality and increased sleep duration in older adults. - Napping can acutely mitigate performance decrements after sleep restriction (e.g., 30-min afternoon nap improved sprint and running performance vs no nap). - Sex differences: females often obtain more total sleep yet report worse sleep and higher insomnia prevalence; they exhibit greater SWA and larger rebound after sleep loss, indicating sex-specific homeostatic responses. - Practical implications: align training and competition with chronotype and time-of-day effects; manage light exposure and sleep hygiene; consider chrono-nutrition and supplement strategies; monitor fatigue and biomarkers across the day to optimize performance and reduce injury risk.

The synthesis indicates that aligning training, recovery, and competition schedules with individual circadian profiles can enhance performance and health. Afternoon peaks in core body temperature and neuromuscular function explain superior anaerobic outputs later in the day, while habitual training can shift these peaks. Hormonal rhythms and sleep architecture integrate with CR to influence anabolism, metabolism, and immune function; thus, protecting sleep and minimizing evening blue-light exposure can preserve melatonin signaling and next-day performance. Nutritional timing and composition (e.g., evening high-GI carbohydrates, tryptophan-rich proteins, melatonin/magnesium/zinc) may improve sleep quality and support recovery. Immune rhythmicity suggests timing considerations for training loads, recovery, and potentially vaccination or anti-inflammatory strategies. Recognizing sex-specific CR and sleep homeostasis can guide tailored interventions. Overall, the findings support chronobiology-informed periodization, sleep management, and chrono-nutrition to address fatigue, prevent non-functional overreaching, and reduce injury risk.

CR is a critical regulator of multiple physiological systems relevant to athletes. Considering chronotype and strategically timing exercise can meaningfully impact performance, with late-afternoon/evening often yielding superior outputs. Light and physical activity act through the master clock to coordinate endocrine, enzymatic, and neurotransmitter systems. Sleep quality and CR shape hormonal (GH, melatonin, cortisol, leptin, ghrelin) and metabolic processes, while the molecular clock also governs immune responses. Nutritional factors (antioxidants, tryptophan-rich proteins, carbohydrates, melatonin, select micronutrients, fruits/vegetables) show potential to improve sleep and support adaptations. Sex differences warrant systematic evaluation. Future research should expand on metabolic circadian clocks, sex-specific responses, and intervention trials integrating training time, light management, and chrono-nutrition to optimize athlete health and performance.

The review notes limited direct evidence on human immune adaptations across exercise and recovery windows in relation to CR; autonomic balance (sympathetic/parasympathetic) interactions with CR were not addressed in depth. There is a lack of studies delineating signaling pathways and molecular/proteomic mechanisms underlying CR influences, including seasonal variation effects. Differences in training models were not comprehensively compared. Nutritional interventions require more rigorous, standardized methodologies and outcome measures to evaluate their circadian effects. As a narrative review, the synthesis may be subject to selection and reporting biases despite incorporating systematic elements.