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# La Luna en Mí: Humans Run on Two Biological Clocks. Only One of Them Got a Calendar
- URL: https://www.kaitlinquinn.com/blog/luna/
- Published: 2026-08-22T04:13:32.000Z
- Updated: 2026-08-22T07:02:54.000Z
- Description: Half the population runs on an unmapped clock. Here's how to chart yours.
- Author: Kaitlin Quinn

Men run on a 24-hour hormonal clock. Women run that same clock plus a second one, roughly 29 days long. Every school day, workweek, and performance review in this country is built around the first clock and blind to the second. We don't treat that as a design flaw. We treat it as a personal failing, and we have a word for the women it happens to: hormonal.

I think that's backward, and I think it's fixable at the individual level starting this month.

I made something tangible out of this research. It's called [La Luna en Mí](https://buy.stripe.com/9B69AS5OZ1xRbkR5GD6sw00?ref=kaitlinquinn.com), an eight-panel field book that fits on a single folded sheet of paper and teaches you to read your own cycle over a single month using the moon as a ruler. There's a small price on it, which is really just a way to support the work. What follows is the research it came out of, the other half of the same project.

Who this is for: all women, everyone who menstruates, and everyone who came from someone who does. That last group is the one I actually want to talk to for a second.

If you're a man reading this, you have women in your life. A partner, a mother, a sister, a colleague, a daughter. Half the population runs on this system, and almost none of you were taught its basics. I'm thinking specifically about the single dad raising a 13-year-old daughter, who has no idea what to say to her. You don't need to be an expert. You need to be someone who doesn't flinch. Getting comfortable with this is not a favor to women; it's a competence, and the men who have it are noticeably better at their jobs and in their relationships than those who don't. Ten minutes of reading gets you most of the way there.

## The number that isn't real

Let's start with the 28-day cycle, because it's the load-bearing lie.

When researchers analyzed more than 600,000 real ovulatory cycles from over 124,000 users, the mean length was 29.3 days, and only about 13% of cycles were actually 28 days. The follicular phase, from period start to ovulation, averaged around 17 days and ranged from 10 to 30\. Even the luteal phase, which clinical guidelines describe as reliably lasting 14 days, ranged from 7 to 17 days.

The single number that represents how most people understand their bodies corresponds to about one in every eight cycles.

I didn't learn that in school, from a doctor, or from an app. I learned it in January, when I finally had time to look.

## The accidental experiment

In December, I left a traditional corporate role. For years, I worked in public safety technology, in a job that ran on a fixed schedule and a fixed affect: same hour, same energy, same steady face, every day, regardless of what was happening inside me. That isn't a complaint about any one employer. It's what the structure asks of everyone in it.

From January through June, I built my own business. No fixed hours, no performance of consistency, and for the first time in my adult life, no external reason to override what my body was telling me on a given day. I started listening on purpose and writing it down.

What I found in two months wasn't new information. It was old information, correctly labeled for the first time. The Tuesday I'd have called unmotivated was day 2\. The week I'd have called "finally getting my act together" was the estrogen rise, arriving on schedule, as it always had. The stretch when I'd describe myself as unusually thin-skinned about feedback was late luteal, every month, predictable within a couple of days.

I'd been reading my own signals my whole life. I'd just been filing them under character instead of endocrinology.

By March, I could feel a season change coming before it arrived. By June, I was planning around it, which is a completely different experience of being alive than reacting to it.

## Two clocks, one schedule

Testosterone levels build during deep sleep, peak in the early morning, decline throughout the day, and bottom out in the evening before rebuilding overnight. In men in their thirties, the level measured at 4 p.m. runs 20 to 25 percent below the same man's 8 a.m. reading. A clean daily loop: high early, tapering by the end of the day, reset, repeat.

Now look at the standard workday. Early start, front-loaded meetings, big decisions in the morning, energy expected to hold flat across five days, every week, indefinitely. Same output on Monday and Thursday. Same affect in January and June.

That schedule is not neutral. It's a nearly perfect fit for a 24-hour hormonal clock.

One thing to be clear about before I go further, because I don't want this read as a grievance. Men are not swimming in research either. The figure I just quoted, the one that appears in clinical guidance and is repeated everywhere, comes from 66 men. Sixty-six. That's the evidence base for the daily rhythm of half the population. Male bodies were studied first and more extensively, but the results remain limited. This isn't a story about one group hoarding the good data. It's a story about how little of it there is, and how unevenly the little we have got handed out.

The rest of us run that clock too, and a second one on top of it: four distinct hormonal phases across about a month, meaningfully changing energy, sleep, appetite, pain sensitivity, and social appetite. Two clocks. One of them is entirely unaccounted for in how work, school, and social life are organized.

We don't ask whether the structure fits. We ask what's wrong with the person who doesn't fit it. She's inconsistent. She's difficult that week. She's hormonal.

## The word

I hate that word, and I want to be precise about why.

We don't call a man hormonal when he's short with a colleague at 4 p.m., with his testosterone down roughly a quarter from where it was at 8 a.m. He's having a day. We reserve hormonal for women, and we apply it in both directions: to sadness and to enthusiasm. A woman who's excited is hormonal. A woman who's certain is hormonal. It doesn't describe a state. It's a device that makes her input feel weightless.

The irony is that the physiology runs the other way. Cyclical variation isn't random. It's patterned, repeating, and forecastable to within a couple of days once you've tracked three cycles.

Here's the claim I'll defend: women aren't more emotional than men. Our hormonal variation is slower, larger, and more legible than theirs, and we've built a culture that treats legible variation as a defect and invisible variation as normal. Give someone the vocabulary and the pattern, and what emerges isn't chaos. It's a forecast.

## Why nobody handed us the vocabulary

Until 1993, women were routinely excluded from clinical research. The FDA had recommended in the late 1970s that anyone who could become pregnant be excluded from early-phase drug trials, and researchers were encouraged but never required to include women. They largely didn't. Even the lab mice were male. It took an act of Congress, the NIH Revitalization Act of 1993, to mandate inclusion in federally funded clinical research.

That law passed the year I was born. I'm thirty-three. Every study of women that the federal government has ever been required to fund fits inside my lifetime, and I'm not old. The foundation of modern medicine was poured before I got here, and women weren't in the mix.

The correction has been slower than you'd hope. A congressionally mandated National Academies review found that only 8.8% of NIH research grant funding between fiscal years 2013 and 2023 went to women's health research, and that the share of the NIH budget allocated to women's health research shrank even as the budget grew. Conditions that fall mostly on women are funded well below what their disease burden would predict.

Hold that against a simpler number: women are 50.9% of the United States population.

I'm not asking for special treatment. I'm asking for proportional treatment. If we're half the population, the research should reflect that. That's arithmetic, not ideology. It's hard to explain away, because fiscal 2013 to 2023 spans administrations of both parties and a global pandemic that put health research on every front page on earth. The share still went down.

Here's the downstream cost, and it's the one I care about most.

When the research doesn't exist, the language doesn't exist. When you don't have language, you can't advocate for yourself. You go into an appointment and say the only thing you've been equipped to say: I don't feel right, and I don't know why. That sentence has a predictable outcome. You get labeled anxious or dramatic or told to manage your stress, and you leave with nothing.

Compare that to: my luteal phase runs for 9 days; my mood drops sharply on days 24 through 27; it's happened every cycle for six months; here's the record. Same body. The only things that changed are the words, the data, and the named pattern.

"Listen to your body" is what you say when you have nothing more specific to offer. It isn't wisdom. It's a thin evidence base handed back to us as a matter of personal responsibility.

## What we do know

Some of this is solid and badly taught. Some of it is repeated everywhere and doesn't hold up. It matters which is which, so I'm going to sort them.

Cervical fluid changes in a predictable sequence. Dry after your period, then creamy, then clear and stretchy like raw egg white as ovulation approaches, then drying again. The sequence tracks the estrogen rise. The timing doesn't run on a calendar, which is the part people get wrong. When the WHO ran a five-country trial involving 869 women, ranging from those who couldn't read to those with graduate degrees, 93% produced an interpretable chart in their first month of trying. Almost anyone can learn to see this. Too few are taught to.

Basal body temperature steps up after ovulation, about half a degree Fahrenheit and sometimes closer to a full degree, and holds until the next period. It's a receipt, not a forecast. It confirms ovulation happened, after the fact, which is exactly the kind of unglamorous, retrospective information that builds a pattern over three months.

The luteal phase warms you and increases your appetite. Core temperature runs a few tenths of a degree higher, resting metabolic rate rises by five to nine percent, and pooled studies indicate energy intake is around 168 calories per day above the follicular phase, with wide variation among women.

Then there's the claim I most wanted to make, and can't. Estrogen's effect on mood and memory is where the popular story runs past the evidence. The largest meta-analysis to date, 102 studies and nearly 4,000 women, found no reliable shift in verbal recall or any other cognitive domain across the cycle. On mood, most prospective studies find no consistent premenstrual pattern in the general population. PMDD is real and disabling and affects a small minority; its existence is not proof that everyone's mood runs on their cycle.

I could have left that out. I'm including it because if I only hand you the parts that flatter my argument, I'm doing to you exactly what's been done to us.

Since the null result is more interesting than it first looks. A population study asks whether women, on average, shift the same way at the same time. Averaging thousands of different women together is very good at erasing a pattern that is real in each of them and differently timed in all of them. "No consistent pattern across women" is not the same finding as "no pattern in you," and only one of those two questions has ever been properly funded.

None of this requires a subscription. It requires knowing what to look for and somewhere to write it down.

One more thing belongs in this section, and it isn't in any journal. There's a second body of knowledge running alongside the published one: what your grandmother knew, what gets passed between women in kitchens and group chats and the backs of cars, carried for generations with no citation attached because for most of those generations nobody offered to study it. Some of it will turn out to be right. Some of it won't. I'm not going to dress it up as science or throw it out, so the field book grades every claim with a moon: full for replicated research, waxing for early work, crescent for what's been carried. Inherited knowledge isn't proof. It's a hypothesis that survived a hundred years without funding, which is more than most hypotheses can say.

## The science being written right now

A research group at UC Santa Barbara ran a study called 28andMe: one participant, daily blood draws, and brain scans across an entire menstrual cycle. Dense sampling, they call it. Instead of comparing many people once, you study one person many times.

They found that estradiol reshapes the coordination of large-scale brain networks and that progesterone measurably alters the volume of the medial temporal lobe, which includes the hippocampus, over the course of a single month. A 2025 follow-up in Nature Neuroscience extended the method to four more people, who were scanned repeatedly across their cycles, and found brain volumes fluctuating in each of them.

Four people. I'm telling you the number because a small study should move you less than a large one, and you deserve to weigh it correctly. But read it as what it is. Dense sampling is expensive; the money to run it across hundreds of women has not been available, and a thin evidence base does not mean there's nothing to find. It's a record of where the money went. These aren't weak scientists. They're the first ones through the door.

The brain isn't a fixed object that hormonal weather passes over. The brain is part of the weather.

That finding exists because someone finally thought to look at one body closely and repeatedly across a full cycle. Which is structurally exactly what I did on paper at my kitchen table, and exactly what the field book asks of you.

## On being your own evidence

I've cited studies throughout so you can check me, and because when you walk into a doctor's office, the currency is data. That's real. It's also not the only thing that's true.

Every woman I know has a body of knowledge about herself that no study has ever confirmed, and she has been quietly taught that it doesn't count. The thing you notice every month. The pattern your mother described to you. The three days you've learned to stop scheduling. It doesn't count as evidence about women, and I won't pretend otherwise; one person's experience can't tell you what's true of the population. But it is the best evidence in existence about you, and you are the person you're making decisions for.

That's the distinction I'd ask you to hold. Not that anecdote is science, and not that anecdote is worthless. That the question "what's true of women" and the question "what's true of me" are different questions, and the second one is answered by exactly the kind of careful, repeated, written-down noticing that the 28andMe team did with a scanner and you can do with a pencil.

It's also the answer to the mood studies. I can't tell you the research says your mood tracks your cycle, because at the population level it doesn't. What I can tell you is that mine does, that I have six months of pages proving it, and that no study will ever be run that's about you specifically. Three cycles of writing it down and you'll know. That's not a consolation prize for missing data. For your own life, it's better evidence than the study would have been.

Research generalizes. Attention particularizes. You need both, and only one of them is currently being withheld from you.

## About the moon

I named it La Luna en Mí, so let me be straight about the evidence.

The tidy version, bleeding on the new moon and ovulating on the full moon, does not hold at the population level. Clue, one of the most widely used period-tracking apps in the world, ran the numbers on 7.5 million cycles from 1.5 million users and found that period start dates fell randomly across the lunar month. That's a company analysis, not a peer-reviewed paper, and I'm citing it on purpose: they're sitting on one of the largest cycle datasets that exist, and the fact that they went looking is worth as much to me as the result. A 2021 study of 529 women in Japan found no association either.

But it isn't closed. A 2021 analysis of women who'd tracked their cycles by hand for up to 32 years found intermittent synchrony with lunar luminance and gravimetric cycles, appearing and disappearing over a lifetime rather than holding steady. A 2024 study of about 3,000 women, published in Science Advances, found a weak but statistically significant association with the moon, with the alignment differing between the European and North American groups. A 2025 follow-up of 176 long-term records found the same pattern, with a weakening after roughly 2010, which the authors attribute to LED lighting and screens flooding our nights with artificial light. Independent experts reviewing that work call the association real and worth investigating while noting there's no established mechanism.

So: weak, contested, possibly a real signal we're drowning out with our own light. Fascinating. Not something to build a practice on.

Which is why the field book doesn't. The moon in it isn't a driver; it's a ruler: a clock face nailed to the sky that needs no battery, no login, and no company's servers, moving at a rate close enough to a cycle that you can read the drift between the two. You don't sync to the moon. You map yourself against it.

[*La Luna en Mí*](https://buy.stripe.com/9B69AS5OZ1xRbkR5GD6sw00?ref=kaitlinquinn.com) *is here if you want it. A few dollars, straight into the next one of these.*

## Why paper

I could have built an app. I build things for a living. Paper was deliberate.

Tracking apps collect some of the most intimate data a person generates and hold it on servers you don't control under terms you didn't negotiate. A pencil mark on a folded sheet in your own house has a very different risk profile. That's a public health position, not a nostalgic one.

Paper also does something that apps are bad at. An app predicts for you, and you learn nothing; it hands you a number and asks you to trust it. Filling in the page yourself is where the learning happens, because you have to look, notice, and name what you noticed. That naming is the entire point. It's how you get the vocabulary nobody handed you.

## The field book

La Luna en Mí walks you through one complete cycle as four seasons: winter for the bleed, spring for the rise, summer for the peak, autumn for the descent. Each season covers what's happening hormonally, what to watch for physically, and how to set up your body, your brain, and your space for the season you're in and the one coming next. You draw the moon four times, on four nights your own body picks.

Every claim carries a small moon mark showing how solid it is. Full moon means replicated science. Waxing means emerging research that is early and still growing. Crescent means tradition, carried but not proven. I'd rather hand you something that admits what it doesn't know than something that flattens folklore and physiology into one confident voice.

After one cycle, you'll know your real cycle length. After three, you can look a month out and know roughly how you'll feel, which means putting the big asks where your energy is and the quiet where it isn't.

Download it, print it at home, fold it once. A field book, not medical advice. Entirely, deliberately, offline. It costs about as much as a coffee, which is less about buying a product than about keeping a research artist in paper and time.

I spent six months outside a schedule built for a clock I don't run on, and I got my own body back. Most people can't take six months off to figure this out. That's exactly why it fits on one piece of paper.

[Get La Luna en Mí](https://buy.stripe.com/9B69AS5OZ1xRbkR5GD6sw00?ref=kaitlinquinn.com)

## Sources

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![](https://storage.ghost.io/c/22/ec/22ec95e5-8838-4888-a1e4-cfba524152e8/content/images/2026/08/feature-c-month-dial.png)