Moon Phases and Menstruation Cycle: Separating Myth from Science

Can the Moon Really Affect Your Menstrual Cycle?

For thousands of years, women have noticed something curious: the average menstrual cycle lasts about 29 days, while the lunar cycle spans 29.5 days. Coincidence? Maybe not. Across cultures worldwide, people have long believed in a connection between the moon and the menstrual cycle.

The words "menstruation" and "menses" themselves come from Latin and Greek roots, meaning "month" and "moon." But does the moon affect menstrual cycle timing in any measurable way, or are we simply seeing patterns where none exist?

The answer sits somewhere between ancient wisdom and modern understanding. While your cycle probably won't perfectly sync with lunar phases like clockwork, decades of research suggest the relationship between moon phases and menstruation is more nuanced than you might think.

The Gravity Factor: When Moon Cycles Still Matter

Despite the loss of population synchrony with visible moon phases, recent groundbreaking research published in 2025 found something remarkable: even in our artificially lit world, the moon and the menstrual cycle still show connections during specific times when gravitational forces peak.

A landmark study analyzing 176 women's menstrual records spanning up to 37 years discovered that menstrual cycles recorded before 2010 showed significant synchronization with the Moon, while those after 2010 mostly lost this connection, except during January when gravitational forces between the Moon, Sun, and Earth are strongest.

The researchers examined three distinct lunar cycles that affect both moonlight and gravity on Earth:

• The Synodic Month (29.53 days): The familiar cycle from new moon to full moon, driven by changing moonlight intensity.
• The Anomalistic Month (27.55 days): The cycle of the Moon's elliptical orbit, affecting gravitational pull as it moves between perigee (closest to Earth) and apogee (farthest).
• The Tropical Month (27.32 days): The Moon's declination cycle, causing variations in tidal forces.

The study found that menstrual cycles could intermittently synchronize with all three lunar cycles, with the synodic month acting as the strongest influence before widespread light pollution.

The Brain Connection: Where Your Cycle Really Starts

At Samphire, we focus on a fundamental truth: when it comes to women's health, the brain is the missing link. Every hormonal change starts in your brain. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which signals your pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones then affect your ovaries, triggering the cascade of events we call the menstrual cycle.

Understanding any potential moon and menstrual cycle connection requires understanding how your brain processes environmental signals. Your brain's circadian system responds to light cycles, regulating sleep, mood, and hormonal patterns. When light cycles are disrupted, whether by artificial lighting, shift work, or other factors, the downstream effects can include menstrual irregularity.

Circadian rhythms affect sleep and the menstrual cycle in interconnected ways. Sleep disruption can worsen menstrual symptoms, while hormonal fluctuations across your cycle can affect sleep quality. Your brain integrates all these signals, making it the true control center of your reproductive health.

Rather than treating symptoms in isolation, Samphire's approach recognizes that your brain orchestrates how your body responds to environmental signals, including light, stress, and hormonal changes.

What Modern Science Says About Individual Variation

While large population studies often find no consistent correlation, individual experiences vary widely. Some women do notice patterns between their cycle and lunar phases, while others see no connection at all.

Several factors influence whether you might experience any synchronization:

• Cycle length: Women whose natural cycle length is close to 29.5 days may be more likely to experience temporary alignment with moon phases and menstruation patterns. The 2025 research showed that women under 35 with cycle lengths close to 29.5 days had the highest likelihood of temporary synchronization.
• Age and life stage: The same study found that younger women showed 23.6% synchronization with the full or new moon on average, while women over 35 showed only 9.5% synchronization. This aligns with the observation that menstrual cycle length tends to shorten with age, moving further from the 29.5-day lunar cycle.
• Light environment: Your exposure to artificial light at night can disrupt any natural synchronization. The research showed that women described as "night owls" showed no synchronization with lunar cycles at all.
• Gravitational timing: Even in modern life, synchronization persists when gravitational forces are strongest, during perihelion in January and during certain 18-year lunar cycles when the Moon's orbit aligns in specific ways.

The key insight from decades of research is that even when synchronization occurs, it tends to be temporary and intermittent rather than consistent and lifelong.

Track your cycle with the Samphire app to understand your unique patterns. Whether or not your cycle aligns with the moon, understanding your individual rhythm helps you manage symptoms and plan your life around your natural fluctuations.

What Decades of Scientific Research Tell Us

Scientists have been studying whether the moon affects menstrual cycles since the 1930s. When we look at this body of research spanning nearly a century, we see contradictory findings that tell an interesting story about how our modern lives have changed our relationship with natural cycles.

Studies Find a Connection

Between 1980 and 1987, researchers conducted several studies examining women whose cycle lengths were close to 29.5 days. The first study in 1980 analyzed 68 women with 29.5-day cycles and found that 69% had their periods between the first quarter and last quarter of the moon phase and menstrual cycle, also known as the "light half" when the full moon occurs. Because it's statistically unlikely for this percentage of women to menstruate during the same lunar phase, the researchers concluded there was a real association.

A follow-up study in 1987 expanded this research, analyzing data from four different groups of women between 1976 and 1983. Women with 29.5-day cycles were more likely to menstruate during days 10 to 20 of the moon cycle, approximately around the time of the full moon and menstrual cycle alignment. However, the effect was modest. For example, about 2.2% of participants menstruated on the new moon, while about 4.4% menstruated the day after the full moon.

A 1981 study confirmed these findings, showing that women with 29.5-day cycles were more likely to menstruate during the light half of the lunar cycle, particularly within 3 days of the first or third quarter. Interestingly, only about one in three women in these studies had cycle lengths matching the lunar cycle.

In 1986, researchers in China analyzed 826 young women aged 16-25 and found that 28.3% of menstruations occurred around the new moon, while only 8.5-12.6% occurred at other times of the month. What's notable is that this study didn't restrict participants to those with 29.5-day cycles, yet still found a pattern.

Studies Find No Connection

However, not all research supports the moon connection. A large 1937 analysis examined menstrual records from thousands of women but found no clear pattern linking menstruation to specific lunar phases. A 1974 study similarly found no relationship, and a 2013 analysis also found no correlation between moon phases and menstruation.

Why the Contradictory Results?

The key insight from all this research is that moon phase and menstrual cycle synchronization appear to depend on several factors:

• Cycle length matters: Women whose natural cycle length closely matches the 29.5-day lunar cycle are more likely to experience temporary alignment. When your cycle length differs significantly from the moon's cycle, synchronization becomes statistically unlikely.
• Age plays a role: Younger women (under 35) with cycle lengths closer to 29.5 days show stronger patterns of synchronization than older women. As women age and cycle lengths shorten, the likelihood of lunar alignment decreases.
• Modern life disrupts the connection: Recent research suggests that before 2010, menstrual cycles showed more significant synchronization with the moon. After 2010, when LED lighting and smartphones became widespread, this synchronization largely disappeared at the population level.

How Light Exposure Affects Your Cycle

Your menstrual cycle is controlled by your brain, specifically the hypothalamus and pituitary gland. These brain regions respond to light exposure, which influences the production of melatonin, a hormone that regulates sleep-wake cycles and may affect reproductive hormones.

Research shows that women working rotating night shifts have more irregular menstrual cycles compared to day workers. Night shift work disrupts natural circadian rhythms, leading to cycle irregularity and menstrual dysfunction. This demonstrates that light exposure directly impacts the brain systems that control your cycle.

One fascinating 1978 experiment found that women with irregular cycles who were exposed to bright light for three nights during specific cycle phases experienced improved regularity. This suggests that the brain's circadian system, which processes light signals, plays a crucial role in menstrual regulation.

Before electric lighting, moonlight was the primary source of nighttime illumination. The full moon can provide up to 0.25 lux of light, potentially enough to influence the brain's light-processing systems. However, today most of us are exposed to artificial light well into the night, from overhead lights to smartphones to LED screens. This constant light exposure doesn't just mask natural moonlight cycles; studies suggest it can actually shorten menstrual cycle length and reduce cycle regularity.

This explains why older studies conducted before widespread electric lighting found stronger connections between whether the moon affects menstruation patterns, while modern large-scale analyses often find no population-level correlation. We're simply exposed to too much artificial light for subtle lunar influences to have consistent effects.

Brain-Based Solutions for Real Menstrual Relief

Whether your periods sync with lunar phases or follow their own rhythm, many women experience challenging symptoms like pain, mood changes, and fatigue. Rather than guessing about lunar influences, you can take concrete action to feel better throughout your cycle.

At Samphire, we've developed hormone-free, drug-free solutions that work with your brain's natural neuroplasticity. Lutea™, our North American product, uses gentle neurostimulation to support your brain's pain and mood regulation systems.

Unlike hormonal treatments that alter your natural cycle or medications with systemic side effects, Lutea™ works with your body's own regulatory mechanisms. Just 100 minutes of use per cycle can help ease menstrual pain and mood symptoms without disrupting your hormones. Lutea™ is not a medical device, but rather a neurotechnology tool designed to support your brain's natural ability to regulate pain and mood.

For women dealing with PMS, PMDD, or dysmenorrhea, understanding your cycle patterns, whether influenced by the moon or not, combined with brain-first interventions, can transform how you feel every month.

Our Samphire app acts as a cycle diary, helping you spot when symptoms are likely, plan for focus days and rest days, and build habits around your natural rhythms. Track patterns over time to see if you notice any personal lunar connections, or simply use the insights to better understand your unique cycle.