Circadian rhythms are the internal 24-hour biological clocks that regulate sleep-wake cycles, hormonal release, metabolism, and other physiological processes. In athletes, circadian alignment is crucial for optimizing performance, recovery, and mental alertness. Circadian misalignment—when the internal clock is out of sync with the environment—can lead to fatigue, poor performance, impaired recovery, and even increased injury risk. Assessing this misalignment requires identifying reliable biological markers, and managing it can be significantly supported through targeted nutritional strategies.Reliable Biomarkers of Circadian Misalignment in Athletes Dim Light Melatonin Onset (DLMO) DLMO is considered the most reliable and precise biomarker for assessing circadian phase. It refers to the time in the evening when melatonin secretion begins under dim light conditions. In a typical circadian cycle, this occurs about 2–3 hours before habitual bedtime. A delayed DLMO is often seen in athletes who travel across time zones (jet lag) or those with late training or competition schedules. Melatonin can be measured through saliva or blood samples, and a shift in its onset time can indicate circadian disruption. Cortisol Rhythms Cortisol, the primary stress hormone, has a strong circadian pattern—peaking in the early morning (around 7–9 AM) and gradually declining throughout the day. In circadian misalignment, such as in overtraining or irregular sleep-wake cycles, the cortisol peak may become flattened or delayed. Salivary cortisol levels at different times of the day can provide insights into the stress response and circadian health of athletes. Core Body Temperature (CBT) Rhythm Core body temperature follows a rhythmic pattern, reaching its lowest point in the early morning and peaking in the late afternoon. A disruption in the timing of this rhythm can reflect misalignment. Continuous monitoring or even oral temperature assessments can help track the phase of the body’s internal clock. A mismatch between temperature patterns and activity schedules can impair performance and recovery. Actigraphy and Sleep-Wake Patterns Wearable actigraphy devices track rest-activity cycles and can give indirect but continuous assessments of circadian rhythm patterns. By comparing actual sleep-wake behavior with expected circadian timing, misalignment can be diagnosed, especially in cases of sleep disorders or irregular training schedules. Clock Gene Expression (Experimental) Emerging research has shown that clock-related genes like PER1, PER2, BMAL1, and CLOCK express rhythmically in peripheral tissues such as blood or skin. Alterations in their expression pattern can signal internal clock disruption. While not yet used widely in clinical settings, these genetic markers are gaining interest for future use in precision sports science.Influence of Nutritional Interventions on Circadian Biomarkers Nutrition is a powerful tool for regulating circadian rhythms and can either worsen or improve circadian alignment. The science of chrononutrition—eating in sync with the biological clock—has demonstrated measurable effects on core circadian biomarkers like melatonin and cortisol. Meal Timing The timing of food intake plays a key role in entraining the peripheral clocks located in tissues like the liver, muscles, and gastrointestinal tract. Consistently eating meals during daylight hours, particularly having breakfast early and avoiding late-night eating, helps synchronize internal rhythms with the environment. Irregular meal timing can desynchronize internal clocks and exacerbate misalignment, especially in shift-working or traveling athletes. Macronutrient Composition Different macronutrients affect circadian rhythms in unique ways. For instance, carbohydrate-rich meals in the evening can promote the release of serotonin and melatonin, which are beneficial for sleep onset. On the other hand, protein-rich breakfasts help stimulate dopamine and norepinephrine production, enhancing morning alertness and focus—key for training or competition. Athletes can use this knowledge to align their nutrition with performance demands. Melatonin and Tryptophan-Rich Foods Natural food sources such as tart cherries, bananas, oats, and milk contain melatonin or tryptophan, a precursor to serotonin and melatonin. Including these in the evening meal can support melatonin production and help restore normal sleep cycles, especially after travel or evening games. Caffeine and Stimulants Caffeine affects adenosine receptors and can delay melatonin secretion if consumed too late in the day. While pre-competition caffeine can enhance performance, it should be avoided after mid-afternoon to prevent disruption of DLMO and sleep patterns. Micronutrients: Vitamin B12 and Magnesium Vitamin B12 is known to influence melatonin synthesis and circadian phase shifts, particularly in blind individuals or those with non-24-hour sleep-wake disorder. Magnesium supports deep sleep and may help normalize disrupted sleep rhythms. These micronutrients, when strategically supplemented, may assist in restoring circadian balance.Conclusion Reliable biomarkers such as dim light melatonin onset (DLMO), cortisol rhythms, core body temperature, and actigraphy patterns provide valuable insights into the circadian health of athletes. These markers help detect internal desynchronization, which can impair performance, mood, and recovery. Nutritional interventions—especially involving meal timing, macronutrient balance, and melatonin-enhancing foods—offer practical strategies to correct circadian misalignment.

✅ Reliable Markers of Circadian Misalignment in Athletes

Melatonin (Dim Light Melatonin Onset DLMO)

It is the gold standard for measuring circadian phase.

Measured by either saliva, blood, or urine (6-sulfatoxymelatomin).

If this late night training or jet lag shifts DLMO it will tell us that the circadian rhythm is delayed or advanced.

Core Body Temperature(CBT) Rhythm

CBT follows a predictable circadian pattern.

Continuous monitoring (e.g., ingestible sensors or wearables) can reveal misalignment.

A delayed or flattened rhythm suggests circadian disruption.

Cortisol Rhythm

Normally peaks in early morning, declines throughout the day.

Salivary cortisol awakening response (CAR) is a practical marker.

Blunted or delayed CAR can reflect circadian misalignment due to late-night training, travel, or sleep disruption.

HRV is not a clock gene marker but rather physiologically linked to circadian stability.

When there is a habitual sleep timing mismatch with the chronotype — social jet lag, it is considered a red flag.

Clock Gene Expression (BMAL1, PER2, etc.)

Taken from peripheral tissues (blood, buccal swabs)

Research-only for now — doesn’t really help in the field

Could be promising in elite settings with high-tech labs

🥗 Nutritional Interventions and Their Effects on Circadian Biomarkers

Meal Timing (Chrononutrition)

Late eating at night blunts melatonin as well as the cortisol rhythm.

eTRF can resynchronize the secretion of melatonin and cortisol with light–dark cycles.

For athletes: meal times matched to natural light could improve sleep and hormonal rhythms.

Macronutrient Composition

High protein breakfast: it helps in morning alertness, also maintaining cortisol rhythm.

High-carb dinner can increase the synthesis of melatonin and the time when one becomes sleepy by increasing the transport of tryptophan across the blood-brain barrier.

Nutraceuticals

Melatonin supplements for phase shifting or re-aligning the clock, jet lag, night competitions.

Magnesium & zinc shown to improve sleep efficiency; may indirectly affect circadian markers.

Tryptophan-rich foods support evening melatonin synthesis.

Caffeine

Though performance-enhancing, caffeine taken late in the day delays melatonin onset.

Should be strategically timed (early/mid-morning use recommended).

Practical Application in Athletes

Monitor: salivary melatonin and cortisol + wearable sleep trackers.

Intervene: through light exposure, meal timing, and nutrient composition.

Align: training and competition schedules with athletes’ chronotype when possible.

Circadian misalignment in athletes can be assessed using several biological markers. The most reliable biomarkers include hormones such as melatonin and cortisol, which naturally fluctuate during the day and night. For example, melatonin increases at night to promote sleep, while cortisol peaks in the early morning to support wakefulness. Any disruption in these patterns may indicate circadian misalignment. Other useful markers include core body temperature, which follows a daily rhythm, and actigraphy, which monitors sleep-wake cycles and physical activity.

Nutrition can directly influence these biomarkers and help correct circadian misalignment. Certain foods, like milk, bananas, and nuts, contain tryptophan, an amino acid that helps the body produce melatonin and improve sleep quality. On the other hand, consuming caffeine or high-sugar foods, especially late in the day, can delay melatonin release and disturb the body’s natural rhythm. Moreover, the timing of meals plays a crucial role: eating late at night can confuse the internal clock and affect hormonal balance.

In summary, monitoring hormones like melatonin and cortisol, along with other physiological signs, can reveal circadian disturbances, and targeted nutritional strategies can help restore proper alignment