Empowering Your Health: New WHOOP Research Reveals Key Insights on the Menstrual Cycle

A groundbreaking study published in npj Digital Health analyzed biometric data from WHOOP members to deepen our understanding of physiological changes across the menstrual cycle. The research included data from over 11,000 members, spanning 45,000+ cycles and 1.2 million days of data, leading to the development of a novel measure: Cardiovascular Amplitude.

Cardiovascular Amplitude quantifies fluctuations in Resting Heart Rate (RHR) and Heart Rate Variability (HRV) across the menstrual cycle, offering a new perspective on how the body responds to hormonal shifts. These insights will enhance future menstrual cycle tracking and coaching on WHOOP, helping members gain a clearer picture of their hormonal health and performance.

Understanding the Menstrual Cycle and Biometric Patterns

Each phase of the menstrual cycle is characterized by distinct physiological changes that influence WHOOP data:

• Menstrual Phase : The cycle begins with menstruation, marked by shedding of the uterine lining. This is typically when RHR is lowest and HRV is highest.
• Follicular Phase : Hormone levels gradually rise in preparation for ovulation.
• Ovulation : An egg is released, and estrogen peaks. This is the fertile window, but factors like hormonal imbalances or overtraining can disrupt ovulation.
• Luteal Phase: Progesterone rises, preparing the body for a potential pregnancy. RHR increases, while HRV declines, reaching its lowest point in the final days of the cycle.

Our research found changes in population-level trends across each phase, confirming that RHR and HRV follow a predictable pattern throughout the cycle.

What is Cardiovascular Amplitude?

Traditionally, menstrual cycles have only been analyzed by their duration, but wearable technology enables real-time tracking of daily physiology. WHOOP researchers developed “Cardiovascular Amplitude”, a measure that quantifies the magnitude of change in Resting Heart Rate (RHR) and Heart Rate Variability (HRV) that naturally occurs across each menstrual cycle. The novel metric calculates how much your daily RHR and HRV vary from the highest and lowest points over the course of a menstrual cycle. We found that the cardiovascular metrics followed consistent trends across the menstrual cycle, with resting heart rate (RHR) at its lowest during menstruation (around days 2-7), then rising and reaching its peak in the luteal phase. HRV shows the opposite trend, with the highest values occurring during menstruation and lowest point a few days before the next period. This measure provides a new way to quantify how menstrual cycles impact cardiovascular function, moving beyond basic cycle tracking.

Key Findings from the Study

On average, resting heart rate (RHR) increased by 2.73 BPM from the follicular to luteal phase, while heart rate variability (HRV) decreased by 4.65 ms. Notably, 93% of participants experienced a positive RHR amplitude, confirming the cycle's predictability in WHOOP data. 

Despite only about 89% of women exhibiting a typical cycle length and just 16% following the textbook 28-day cycle, WHOOP metrics captured a consistent physiological pattern across diverse cycle lengths, highlighting the potential of biometric tracking to provide valuable insights into cycle regularity and hormonal fluctuations.

Factors That Influence Cardiovascular Amplitude

WHOOP researchers identified key variables that affect the degree of change in RHR and HRV across the menstrual cycle:

• Age:
  • Finding: Older women experienced reduced fluctuations in RHR and HRV, with an estimated 0.5 BPM reduction in amplitude per decade.
  • Probable Cause: This may reflect age-related shifts in fertility or perimenopausal transitions, which can impact cardiovascular and autonomic function.
• Baseline Fitness:
  • Both extremely high and extremely low RHR and HRV values were associated with diminished cardiovascular amplitude.
  • Overtraining, extreme high or low body weight, or insufficient physical activity can disrupt hormonal balance, leading to irregular menstrual cycles. 
• Hormonal Birth Control:
  • Members using hormonal birth control had significantly lower cardiovascular amplitude, with less than half a BPM change in RHR and a less than 1 ms change in HRV.
  • Since birth control pills create steady hormone levels, these pills may in turn keep RHR and HRV steady across the menstrual cycle.

Even after adjusting for baseline demographics, these patterns remained consistent across the study population.

Why These Findings Matter

WHOOP has identified a new way to understand hormonal health through physiological patterns. While more research is needed, these insights suggest that Cardiovascular Amplitude could help:

• Detect potential cycle irregularities early when significant deviations occur.
• Identify abnormal or anovulatory cycles (when ovulation does not occur) in individuals with low cardiovascular amplitude.
• Monitor changes over time, helping members track age-related hormonal shifts.

These findings reinforce the WHOOP commitment to providing deeper, data-driven insights into women’s health—helping members better understand their bodies and optimize their training, recovery, and well-being.