Your cycle is the fifth vital sign: Why elite athletes track with Clue

At the highest level, performance is built on marginal gains. Athletes already track training load, nutrition, sleep, and recovery—but one key variable is often left out of the equation: the menstrual cycle.

More than a cycle: A window into systemic health

The menstrual cycle is increasingly recognized as a “fifth vital sign”, a real-time indicator of hormonal health, energy availability, and overall physiological function (1,2). 

Beyond sports, it is gaining traction in preventive and metabolic health as a real-time signal of how the body is responding to cumulative stress, including cardiovascular strain (3,4).

For elite athletes, this makes the menstrual cycle more than a monthly event. It becomes a dynamic marker of how the body is coping with training load, recovery, and energy demands.

Hormones, training, and performance

Across the menstrual cycle, levels of estrogen and progesterone rise and fall. These hormonal shifts influence multiple systems relevant to training and performance. 

• Estrogen is associated with muscle repair, mitochondrial function, and potentially improved endurance and recovery (5)

• Progesterone can increase core temperature and cardiovascular strain, particularly during exercise in the luteal phase (6)

These fluctuations can affect:

• Energy metabolism (7)

• Fatigue and recovery (5)

• Thermoregulation (6)

• Sleep and circadian rhythms (8)

• Pain sensitivity and inflammation (9)

• Cognitive function, mood, and motivation (10)

Importantly, these effects are not uniform. The same hormonal changes can feel very different from one person to another, and even from one cycle to the next. This is why understanding your own patterns matters more than relying on generalized advice. 

From averages to excellence: Why your cycle data matters

For decades, sports science has been built on a male-default model, with only ~6% of studies focusing exclusively on female participants (11). This has led to generalized recommendations, such as “cycle-syncing,” that often fail to reflect the complexity of female physiology.

At the same time, the real-world impact is clear. Among athletes not using hormonal contraception:

• 77% report that menstrual-cycle-related symptoms negatively impact their performance (12)

• 74% report disruptions to training availability, including missed or modified sessions (13) 

• 31% of elite athletes experience menstrual dysfunction (14)—an early warning sign for Relative Energy Deficiency in Sport (RED-S) (2)

That’s why your own data matters more than population averages. Research also shows there’s no universal ‘optimal cycle phase’ for strength, endurance, or competition readiness (15). 

Instead, responses are highly individual.

What matters are your personal patterns. 

Tracking your cycle over time can help you understand how your body responds to training, stress, and recovery—so you can identify patterns that are meaningful for you.

Menstrual dysfunction and RED-S: A critical signal

For athletes, the menstrual cycle is not just about symptoms—it’s also a key indicator of health and energy availability.

Menstrual dysfunction (such as irregular or missed periods) is common in elite sport and is often linked to low energy availability. This is when energy intake does not meet the demands of training and daily function (2).

This can progress to Relative Energy Deficiency in Sport (RED-S), a condition that affects multiple systems, including:

• Bone health (increasing risk of stress fractures)

• Immune function

• Metabolic rate

• Cardiovascular health

• Recovery and adaptation (2)

Because hormonal function is sensitive to energy availability, the menstrual cycle often acts as an early warning system. Tracking changes such as longer cycles, missed periods, or shifts in symptoms can help identify issues before they impact performance or long-term health (2,3). 

Turning awareness into action

Cycle awareness allows athletes to move from reacting to symptoms to making informed decisions.

• Training & load: Align perceived effort with performance data to better understand when to push and when to prioritize recovery

• Competition readiness: Use past patterns to anticipate symptoms in the days leading up to competition and prepare accordingly

• Injury risk & recovery: Identify periods of increased fatigue or strain and adjust training to reduce risk

Wearables and cycle tracking

Most wearables provide raw physiological data—temperature, sleep, heart rate—but lack the hormonal context to explain it. 

By combining wearable data with menstrual cycle tracking, athletes can identify:

• Patterns in sleep, recovery, and energy across the cycle

• Temperature shifts linked to ovulation and hormonal fluctuations

• Relationships between heart rate variability (HRV), resting heart rate, and recovery status

• Interactions between sleep, mood, and overall readiness

This turns isolated data points into contextualized, actionable insights.

Closing the research gap

Clue partners with leading institutions to advance female-specific performance research.

By tracking with Clue and consenting to share anonymized data, athletes become active contributors to this progress—helping generate the data needed to better understand female physiology and improve outcomes for the next generation.

Examples include:

• IMPAcT Study (Harvard University) — analyses how cycle phase, age, and contraceptive use shape real-world physical activity patterns, helping reframe how we understand performance, energy, and movement.

• EndoFit Project (National Institute of Sport, Expertise, and Performance (INSEP)) — studies how physical activity relates to symptom experience in individuals with and without endometriosis, contributing to a stronger evidence base for how movement can support symptom management.

The takeaway

Your cycle doesn’t determine your performance—it provides the context for it. Tracking your cycle with Clue helps you understand how your body responds to training and stress, detect early signs of imbalance, and make more informed decisions.

In a field defined by marginal gains, that context can make a measurable difference.