Every woman can attest to the hormonal fluctuations across the menstrual cycle. But what drives these changes, and perhaps more importantly, what makes the hormonal ups and downs and the natural cycles more seamless, regular, and thus easier and healthier? We will answer all of this and much more. 

The menstrual cycle flow

This monthly rollercoaster is an elegantly orchestrated dance of hormones, physical changes, and mood and behavior shifts in healthy, ovulating women. Picture it as a 28-day (give or take) story of natural cycles with distinct chapters, each bringing its own vibe.

It all kicks off with day 1: menstruation. That’s when the uterine lining says, “Time to go!” and sheds, causing your period. Behind the scenes, a hormone called follicle-stimulating hormone (FSH) is waking up. It nudges the ovaries to start growing small sacs called follicles, each cradling an egg. As these follicles get busy, estrogen levels begin to climb.

Enter the follicular phase, or the “getting ready” stage. Estrogen is like the hype girl here, thickening the uterine lining to prepare for a possible pregnancy. You might feel a boost of energy, a little flirty, or just more “on” during this time—estrogen’s got that glow-up effect. This phase rolls on for about two weeks until the big moment: ovulation.

Around mid-cycle (day 14-ish), luteinizing hormone (LH) swoops in with a dramatic surge. It can feel like the womb switching into a different mode. It’s the green light for one lucky follicle to release its egg—a process called ovulation. This is peak fertility time, and some folks notice a twinge of pain (hello, ovary action) or feel extra confident, social, attractive, or even adventurous. Hormonally, estrogen’s still riding high, but soon, it’s progesterone’s turn to shine.

Next up is the luteal phase, the “waiting game.” After the egg pops out, the empty follicle transforms into a temporary hormone factory called the corpus luteum. It pumps out progesterone, which keeps the uterine lining plush and ready, just in case a fertilized egg shows up. Behaviorally, you might feel calmer or more focused early on, but as this phase winds down, progesterone and estrogen levels start to dip if there’s no pregnancy. That drop can bring on the infamous premenstrual vibes—think mood swings, irritability, bloating, cravings, etc. 

If the egg isn’t fertilized, hormone levels crash, and the cycle wraps up where it began: menstruation. The uterine lining sheds and you might feel tired, crampy, or emotional as your body resets for the next round.

So, there you have it—a hormonal dance of FSH, LH, estrogen, and progesterone, with each step shaping how you feel, act, and flow through the month [1]. It’s a wild, beautiful, and natural ride.

What drives these natural cycles?

The lunar cycle clocks in at about 29.5 days—eerily close to the average menstrual cycle of 28-ish days. This has sparked curiosity forever: Could the moon, a legit zeitgeber (time-cue) for tides and critters, also sync up human rhythms?

One study spotted a faint link between lunar phases and menstrual timing across European and North American women [2]. It’s not a tight lockstep—sometimes ovulation vibes with the full moon, sometimes not—but the nudge is there.

How might this work? One theory is light. The moon’s glow, especially at its fullest, could tweak melatonin (your sleep hormone), which chats with reproductive hormones like estrogen and FSH. Another angle? Gravity. The moon’s pull shifts tides, so maybe it subtly sways bodily fluids or ovarian action [3].

Way back, our ancestors lived by moonlight—full moons would have meant more hunting and socializing, syncing fertile days to active nights. Today, artificial light pollution is drowning out the night sky, and the subtle changes of moonlight make the moon a background dancer rather than the choreographer [4]. The real star? An internal, near-monthly clock, with the lunar cycle as a soft nudge [2].

This internal clock evolved to tick close to the 29.5-day lunar rhythm. This idea comes from ocean-edge critters whose breeding is in sync with the tides [5]. Even for us land-dwellers, this ancient clock might linger from our evolutionary past. Maybe it’s an old oceanic beat still humming in our genes.

Interestingly, weekly rhythms give the menstrual cycle a surprising twist! Menstruation tends to peak on Thursdays and Fridays, showing a subtle but real 7-day (circaseptan) rhythm. This pattern pops up across ages, seasons, and lunar phases, especially in cycles lasting 27-29 days [6]. How come? This is likely driven by our modern-day work-week, showcasing the impact lifestyle has. 

Seasonal rhythms also play a role. In summer, compared to winter, FSH secretion increases, ovarian follicles grow bigger, ovulation is around 97% (vs. 71% in winter), and the menstrual cycle is about 1 day shorter. If more sunshine is experienced 2-3 days before ovulation, the overall cycle length is shorter. In other words, summer’s extra light juices up hormones and tightens the rhythm, while the lack of light in winter slows the tempo [7].

Since yearly rhythms have been shown to depend on daily ones [8] and tidal rhythms might stem from a clock that handles both tides and daily rhythms [5], it’s clear that circadian rhythms also shape our monthly cycles.

So what’s driving your menstrual cycle? It is a combination of an internal clock (related to tides and daily rhythms) and weekly, lunar, and seasonal cycles. But the story does of natural cycles not end there. The 24-hour clock, or circadian rhythm, orchestrates the menstrual cycle in no small way, linking day and night to the 28-day rhythm.

Circadian rhythms and the menstrual cycle

Since hormonal changes and their various effects (biological, emotional, behavioral) are the hallmarks of menstrual cycles, where do we look for hormonal regulation?

Daily rhythms are the foundation of all hormonal rhythms. Circadian rhythms regulate daily sleep-wake cycles, metabolic processes, hormonal ebb and flow, and, overall, the timing of all bodily functions [9, 10].

That is true for men and women and, in fact, all animals and insects. The day-and-night cycle sets the main rhythm for life, and hormones fluctuate around this main synchronizer. Interestingly, according to the latest research, women show more circadian clock gene expression than men [11]. 

The clock structure was conserved across sexes and age groups. However, overall gene expression rhythms were highly sex-dimorphic and more sustained in females.

Talamanca, Lorenzo et al

Each phase of the menstrual cycle is under the influence of the circadian timing system [12, 13]. Molecular and genetic biology have confirmed the absolute importance of circadian rhythms in female reproductive health [14]. At the same time, the menstrual cycle interacts with circadian processes to alter the expression of hormonal rhythms and sleep organization at different menstrual phases [1].

For daily functioning, hormones like cortisol, melatonin, GH (growth hormone), insulin, and reproductive hormones (FSH, LH, estrogen, progesterone) sync to light-dark cues via the master clock in your brain. Thus ensuring you’re alert when you need to be, rested when it’s time to recharge, and metabolically balanced around the clock. Cortisol and insulin power your day, and melatonin and GH repair you at night.

For healthy menstrual cycles, circadian rhythms are the unsung heroes. They fine-tune the hypothalamus-pituitary-ovarian axis, where FSH and LH kick off follicle growth and ovulation. Melatonin not only regulates your entire nightly rhythm but also interacts with your reproductive hormones; for example:

• FSH and LH, the hormones that initiate follicle growth and ovulation
• estrogen and progesterone receptors in the ovaries and uterus.

Cortisol and insulin stabilize the energy demands of ovulation and uterine preparation. Hence, if circadian clocks are disrupted, the hormonal dominoes will fall, leading to irregular cycles, fertility issues, and menstrual disorders [15]. Circadian disruption is in the spotlight for its crucial role in the growing rates of infertility for both sexes [16].

In short, circadian rhythms are the daily pulse that keeps both your day-to-day groove and your monthly flow in harmony.

Disrupted circadian rhythms

Most people are unaware of what constitutes a circadian disruption. Here are a few examples of common circadian disruptions:

• not exposing yourself to natural light first thing in the morning,
• not spending enough time outside, 
• bathing in artificial light after dark, 
• skipping breakfast, eating late, and 
• changing sleep/wake times (e.g., sleeping in on the weekend). 

Lifestyle choices such as these, as harmless as they might seem, undermine every system in your body, mess with your menstrual cycles, decrease fertility, and increase rates of miscarriage [17]. Let’s dive into some details. 

Shift work and the menstrual cycle

Shift workers experience the worst kind of circadian disruption. How do a changing schedule and being active at night impact your natural cycles? Buckle up.

• night shifts, rotating schedules, and jet lag are linked to irregular and longer cycles, wonky LH and FSH levels, higher chances of pre-term birth, and lower fertility overall [15, 18, 19]
• the odds of having a menstrual disorder are 454% higher among shift workers compared to non-shift workers [20] 
• night shift suppresses ovarian function by affecting the circadian rhythm of melatonin and prolactin [21]
• women working night shifts have a much higher rate of irregular menstrual cycles compared to those working day shifts [21]
• night shifts significantly increase the risk of irregular menstrual cycles and missed periods [22] 
• flight attendants experience more frequent and worse menstrual pain than teachers [23]
• rotating shift work increases menstrual cycle irregularity [24, 25]
• shift workers experience higher rates of infertility and menstrual disruption (cycles less than 25 days or greater than 31 days) compared to non-shift workers [26]

Here is a summary of the effects shift work has on menstrual cycles and reproductive function:

Light at night and the menstrual cycle

What is one of the main characteristics of shift work? Artificial light at night and being active when we are meant to rest. Light at night is not just a problem for shift workers. Light after dark, especially blue and green wavelengths, is a substantial circadian disruption that’s affecting almost everyone these days. For more information on this topic, read the chapter “Artificial Light” in the learn section of the Circadian app and check out these articles: Ultra-processed light, Optimal light environment and hacks for shift workers.

Let’s look at some studies highlighting the connection between light at night and menstrual disruption:

• light at night is associated with menstrual cycle irregularity, ovulation disorders, significant changes in melatonin and FSH levels [27, 28, 29, 30]
• light at night led to an abnormal follicular phase in 96% of the women, followed by luteal (64%) and ovulation (58%) phase abnormalities. The establishment of a darkened sleeping environment was associated with a significant reduction in abnormal parameters [31].
• light at night has been repeatedly shown to reduce the cycle length for women with irregularly long cycles [32, 33, 34, 35]. Keep in mind that light at night is a circadian disruption and has many negative side effects. If you want to curtail abnormally long cycles, expose yourself to more natural light during the day instead. 
• the circadian timing of women is more sensitive to bright light at night, leading to more significant melatonin suppression than in men [36]
• night owls have a higher risk of cramping (220%), cycle irregularity (90%), and longer overall cycles (170%) [37]
• light at night should be avoided for optimal reproductive health [38] 
• and to drop a little bomb here: high exposure to artificial light at night increases your risk of death by 21-34% compared to minimal exposure [39]

The most direct impact of light at night is on pineal melatonin. Hence, let’s look at a few aspects of this crucial hormone. For a deep dive, pls check the learn section on melatonin in the Circadian app.

Melatonin and the menstrual cycle

Melatonin controls repair and regeneration activities, sets your circadian clocks, regulates sleep, and controls many other essential physiological processes. Melatonin synthesis is suppressed under light at night, and this suppression messes with your immune system and sleep, increases chronic disease & cancer risk, and can desynchronise every cell of your body [40, 41, 42]. The degree of melatonin suppression depends on the frequency, duration, and brightness of light [43, 44, 45]. In other words, light at night undermines in ways you cannot imagine. 

Regular circadian rhythms and cyclic melatonin availability are beneficial in assuring optimal reproductive physiology. Melatonin, a potent antioxidant, is also produced in the ovary. It helps oocyte maturation, shields it from free radicals during ovulation, and boosts corpus luteum function and progesterone levels [38]. 

The level of melatonin in healthy women does not differ across the menstrual cycle [46, 47, 48, 49], but women with premenstrual syndrome (PMS) and premenstrual dysphoric disorder (PMDD) present lower melatonin levels, elevated nighttime core body temperature, and worse sleep quality [50, 51, 52].

Women with polycystic ovary syndrome (PCOS) have altered melatonin levels compared with women without PCOS, with reports of higher serum and urinary melatonin levels [53, 54, 55, 56] and lower melatonin levels in follicular fluid [55, 57].

There is good evidence that melatonin can be used to treat menstrual cycle irregularities, hyperandrogenism, and fertility issues [58, 59, 60], further cementing its crucial regulatory role in reproduction and as an anti-cancer agent [61].

That does not mean buying melatonin pills but making sure not to tank your in-house melatonin production. How? By blocking artificial light at night with blue blockers (VivaRays, BlockBlueLight) and taking charge of your light environment in general. Sleeping in darkness helps to stabilize all the hormonal phases of the menstrual cycle [31].

Sleep and menstrual cycles

Your capacity tomorrow hinges on the quality of your sleep tonight. If your sleep is suboptimal, so is your health, rhythm, and menstrual cycle.

The menstrual cycle interacts with circadian processes to alter the expression of hormonal rhythms and sleep organization at different menstrual phases [1]. The most consistently observed menstrual cycle-related changes in the sleep profile of healthy women are a reduction of REM sleep [62, 63, 64] and a robust variation of sleep spindle activity across the menstrual cycle [65, 66].

Sleep complaints commonly occur in healthy women during the post-ovulatory luteal phase [67]. These complaints reach a higher severity in women suffering from premenstrual dysphoric disorder (PMDD) [68].

Here are some more findings on sleep and the menstrual cycle:

• a longer sleep duration is associated with better regularity of natural cycles [69] 
• women sleeping less than 6 hours report more abnormal (short or long) menstrual cycle lengths [70]
• sleeping in on the weekends (also termed social jet lag, is a very common circadian disruption) is a significant factor associated with severe menstrual symptoms [71] 
• sleep architecture results point to circadian rhythm disturbances in women with menstrually related mood disorders (MRMDs): premenstrual syndrome (PMS), premenstrual dysphoric disorder (PMDD), and premenstrual exacerbation (PME) of other psychiatric disorders [72]

I trust it is becoming obvious by now how everything is connected and influences each other. Sleep, light, darkness, circadian rhythm, hormones, menstrual cycle, and mood are all linked.

Mood and natural cycles

Mood and sexual behaviour vary most substantially over the course of your natural cycle [73]. Mood also varies across the day, the week, and the seasons [74].

Why do you think you feel more gloomy in winter or down when the sun doesn’t shine for days? Because different frequencies of sunlight kick your internal pharmacy into action. For example, you make beta-endorphin, serotonin, and dopamine when your skin absorbs UV-A rays [75, 76, 77, 78].  

A lack of sunlight and, thus, these feel-good hormones are seen in people with anxiety and depression [79, 80], drive seasonal affective disorder [81], and can lead to increased substance abuse [82, 83]. Not to mention that circadian rhythm disruption is a key driver in depression [84, 85]. 

Depressed mood, anxiety/tension, anger/irritability, and sleep disturbances are seen in women with PMDD, a mood disorder affecting 3–8% of North American women [86]. Women with PMDD have lower melatonin levels [30, 51] and reduced plasma [87] and whole-blood serotonin levels [88].

It has been shown that serotonin increase improves premenstrual irritability and dysphoria [89]. You can simply spend more time in the sun to raise your serotonin levels. 

Did you know serotonin is a precursor for melatonin? Hence, you produce more serotonin and, consequently, make more pineal melatonin if you spend more time outside. It’s always a win-win if you play by nature’s rules.

More sun exposure also means you make more vitamin D when UV-B rays are present. How is that relevant to natural cycles, you wonder?

Low vitamin D status has been linked to long menstrual cycles [90], irregular cycles [91], and delayed ovulation [92]. Women with lower vitamin D status also produce less estrogen throughout the menstrual cycle [93].

Thus, bright, sun-filled days improve your mood, menstrual cycles and overall health. Combine that with darkness at night to optimize your mood and circadian rhythm. After all, greater night-time light exposure is associated with increased risk for major depressive disorder, generalized anxiety disorder, PTSD, psychosis, bipolar disorder, and self-harm behavior [94]. Let’s not do that! 

Meal timing and menstrual cycles

We need to touch upon meal timing as well, especially since skipping breakfast has gained such popularity nowadays. Unfortunately, as good as the intentions often are, skipping breakfast is associated with various negative health outcomes, particularly in women [95]. The impact of this behaviour can negatively impact circadian health, metabolism, hormonal balance and more [96, 97]. Breakfast plays a role in regulating the release of various hormones, such as cortisol and leptin, both of which influence fertility. Lets examine a few studies in relation to breakfast and menstrual cycles:

• female college students who did not eat breakfast experienced a significantly higher incidence of period pain than those who consumed breakfast [98]
• skipping breakfast leads to a significantly greater incidence of irregular menstrual cycles [99, 100]
• irregular breakfast consumption can disrupt ovarian and uterine functions, potentially leading to reproductive disorders [95]
• female college students who skip breakfast have a higher incidence of period pain and menstrual irregularities [101]
• skipping breakfast has been linked to insulin resistance and impaired glucose metabolism, both of which are critical for maintaining regular ovulatory cycles [102]
• for women with polycystic ovarian syndrome (PCOS), regular breakfast consumption is associated with improved insulin sensitivity and ovulatory function [103]

A regular breakfast combined with an early dinner (well before bedtime) is in the best interest of women and men alike. Let’s follow nature and reap the benefits.

Daily actions for optimal circadian and menstrual rhythm

I will only list the most critical aspects here without going into the details. The overall key is rhythmicity, meaning showing up every single day. Refer to the Circadian app and our blog for more information.

1. Wake at the same time, but gradually shift times with the seasons (our app does this for you) 
2. Get a big dose of early morning light in your eyes, ideally around sunrise
3. Eat a hearty breakfast within 2-3 hours of getting up
4. Stop eating 3-4 hours before going to bed
5. Have natural light breaks during the day and spend more time outside, exposing your eyes and skin to sunlight (you can set light break reminders in our app)
6. Minimize & block artificial light after dark with blue blockers (VivaRays, BlockBlueLight) and circadian-friendly lighting
7. Go to bed at the same time, but gradually shift times with the seasons (our app does this for you) 
8. Create a dark, tech-free sleeping space

Following these golden rules for optimal circadian rhythms will benefit every aspect of your health. Here is what some of our app users shared on how implementing circadian principles affected their natural cycles:

“I am noticing a really huge difference with my hormones, and specifically with my monthly cycle. Following the Circadian app’s suggestions, I’ve had no PMS symptoms, very little fatigue, and no cramping. I’ve had awful cycles for years. This is a huge deal to be healing and feeling so much better in such a short amount of time.” – Adrienne, USA

“Very practical, keeping my cycles on time has changed my life.” – Laura, Spain

Going with the menstrual ebb and flow

1. Menstruation, a time for introspection, rest, self-care, intuition

Menstruation is not only a process of “shedding” but of circadian autophagy and renewal through the recycling of old cells and proteins in order to create new ones [104]. Hence, the body has a natural inclination to rest during this time. Allow time to retreat from the world, be less social, and practice rest, comfort, and self-care. The mind will tend to be much quieter and emotions more settled. Hence, it is a great time to be quiet and meditate/reflect. 

If possible, refrain from pushing yourself or putting additional stress on your body. Exercise should be gentle in the form of walking, yoga, stretching, etc. Make sure to stay well-hydrated.

By the end of menstruation, your body will have reached a “normal” baseline of hormones and oxidative stress, suggesting the beginning of one of the easier stages of the menstrual cycle, where the body has a relatively lower demand on its antioxidants (including melatonin) compared with the rest of the cycle [105]. 

At her first bleeding, a woman meets her power. During her bleeding years, she practices it. At menopause, she becomes it.

Lucy H Pearce*

On a personal note, I feel that menstruation is a sacred time—a time of honoring the womb, lifeblood, and change. As a man, I want to be of full service to my woman when she bleeds, to cherish her, and to support her needs. Let’s learn to respect, honor, and embrace life, natural cycles, and each other again. All seasons are beautiful.

2. Proliferative phase, a time for action, creativity, new beginnings and energy output

Have work/study to take care of? This is the ideal time to knock things out of the park. It’s also a great time to engage in more intense physical activity.

A big dose of early morning natural light may be even more important during this phase because the morning circadian surge is what orchestrates the morning and early afternoon release of sex hormones. Sex hormones need to be climbing steadily through this phase leading up to ovulation. Ovulation depends on a surge in luteinizing hormone (LH), which occurs at the end of your sleep and requests high circulating estradiol. This shows a direct coupling of your circadian health and your menstrual health [106].

3. Ovulation, a time for communication, collaboration, attracting & magnetizing

Use the increased energy to get out there and get things done. It’s a great time for intense exercise, more social time, mingling with people, and sexual encounters (with yourself or others).

Also, note that ovulation creates higher metabolic demands, which in turn creates extra metabolic products, some of which are considered inflammatory. The body responds to these inflammatory products with antioxidants, such as glutathione and melatonin. This means if you are already prone to inflammation, these conditions may be worse around and after ovulation since there is increased demand on your supply. This is why focusing on antioxidant foods and doubling down on circadian health (i.e., bright days and dark nights) around this time can be especially helpful.

It’s advisable to avoid prolonged fasting during this phase. It can be helpful to support your estrogen metabolism by consuming foods rich in polyphenols, probiotics, and prebiotics.  

4. Secretory phase, a time for being calm & steady, slowly turning inward and wrapping things up

Sensitivity to cortisol is heightened during the luteal phase, so prioritizing self-care and rest is important. Opting for low-impact exercise and a nourishing diet, as well as reducing alcohol and caffeine, can reduce stress on the body.

Give any negative thoughts and feelings space and time to move through. Track the themes that come up at this time (eg, complaints about a partner’s behaviour). Notice if something repeats month after month (it wants you to respond to it). 

Many women have such high circulating hormones between ovulation and menstruation that some of the hormones seem to lose their circadian rhythmicity and can be measured high all the time [107]. It’s one of the reasons this stage of the cycle can feel intense, especially if we live disconnected and out of rhythm.

Learn to expand and breathe rather than contract and react if emotions are strong. Your body produces fewer pleasure hormones (i.e., serotonin) [108], so consciously do more pleasurable things, like hot baths, saunas, massages, chocolate, etc. And, of course, get plenty of sun to boost endorphins, serotonin, and dopamine.

If we live more in harmony with our environment, the premenstrual phase is likely to be a slow, low-key and inward time. In other words, look after yourself, go easy and appreciate the different flavours of your natural cycles.

Wrap up

I hope this sheds some light on why circadian health is foundational for healthy menstrual cycles and what daily actions optimize both circadian and menstrual rhythms. Doing so will not only create more ease with the menstrual ebb and flow but also support and elevate every aspect of your health and well-being, including fertility [16]. 

Here is to creating more harmony with the daily, monthly, and seasonal natural cycles. Let’s align our behaviors with nature for the benefit of all. 

Mind your rhythms, mind your light, see you outside.