Cyclical Patterns in Human Biology: What Science Knows (and Doesn't)

In 1975, a French cave explorer named Michel Siffre spent six months underground in Texas with no clocks and no natural light. When he finally emerged, blinking into the sun, he was confident about one thing: he had been down there for 151 days. The actual number was 179. He had lost 28 days — because without light cues to anchor him, his body's internal clock quietly drifted to a 25-hour cycle instead of 24, compounding further from the outside world with every single loop.

Siffre's experiment accidentally revealed something profound about cyclical patterns in human biology: your body keeps time whether you track it or not. And it doesn't run just one clock. It runs dozens of them — stacked, overlapping, cycling on timescales from 90 minutes to 28 days to an entire year — governing everything from your mood to your immune function to the quality of your deepest sleep.

Three Layers of Biological Time

The clearest way to understand the body's internal timing is through three categories, each defined by how long its cycle takes. According to research summarized by Matthew Lees, PhD, at Troscriptions, biological clocks drive or alter sleep patterns, alertness, mood, physical strength, and blood pressure — in fact, most aspects of physiology and behavior fall under their influence.

Circadian rhythms are the ones you've probably heard of — cycles that repeat approximately once every 24 hours. Your sleep-wake cycle is the most obvious example, but circadian rhythms govern gene expression, enzyme activity, metabolic processes, and hormonal fluctuations throughout virtually every cell in your body.

Ultradian rhythms cycle faster than 24 hours — sometimes every 90 minutes, sometimes every few hours, sometimes in bursts of seconds. These sub-daily rhythms govern things you rarely think about: the stages of your sleep, pulses of growth hormone, even the alternating dominance of your nostrils (yes, your nose switches sides roughly every 2.5 hours).

Infradian rhythms cycle slower than 24 hours — from a few days to many months. The menstrual cycle, running approximately 28 days, is the primary human example. Seasonal affective disorder — the real, documented mood shift that follows the arc of the year — also qualifies as an infradian rhythm according to the Cleveland Clinic.

All three are running simultaneously, right now, in your body.

Why Your Body Runs a 25-Hour Clock (Not 24)

Here's the thing Siffre discovered by accident, and that Aschoff and Wever confirmed in 1976 by sealing participants in a WWII bunker with no natural light or social cues: the human circadian clock, left to its own devices, defaults to approximately 25 hours, not 24. Some people's free-running clocks ran even longer — up to 29 hours. Without external anchors, we naturally drift.

This is why light matters so much. Light is what chronobiologists call an exogenous zeitgeber — an "external time-giver" — and it's the primary force that resets and re-anchors your internal clock to the real world each day. When morning light hits the retina, it travels via the optic nerve to a cluster of roughly 10,000 neurons in the hypothalamus called the suprachiasmatic nucleus (SCN) — the brain's master circadian pacemaker. The SCN signals the pineal gland to suppress melatonin, triggering wakefulness. At night, the process reverses.

This is also why jet lag feels brutal and why shift work is genuinely hard on the body. When your environment sends timing signals that conflict with your inherited clock period, you enter what researchers call phase misalignment — and it's been described as a public health concern. The cognitive effects are real and measurable.

Every Organ Has Its Own Clock

Here's where it gets truly wild: the SCN is only the master clock. Nearly every organ in your body has its own peripheral circadian rhythm running in parallel — the lungs, the liver, the pancreas, the spleen, the thymus, the skin.

The liver, for instance, doesn't just run a 24-hour circadian cycle. It runs a 12-hour ultradian rhythm, with over 200 genes cycling on and off at sunrise and at sunset — two distinct metabolic "rush hours" every single day. This means your liver is doing fundamentally different biochemical work at 7am than it is at 7pm, regardless of what you've eaten or how you feel.

The Per3 gene plays a role in synchronizing both the central SCN clock and these peripheral organ clocks. When all these systems are aligned with each other and with the external world, everything hums. When they fall out of sync — from irregular sleep, constant time zone changes, or extended blue light exposure at night — the downstream effects ripple across metabolism, cognition, mood, and immune function.

The 90-Minute Wave You Ride Every Night

The ultradian rhythm most people have unknowingly experienced is the REM-NREM sleep cycle. In 1957, Dement and Kleitman used EEG measurements across 61 nights with nine participants to map it precisely: sleep is not one continuous state but a series of distinct stages cycling approximately every 90 minutes, repeating 3–5 times in a normal sleep episode.

Each cycle moves through lighter NREM stages into deeper slow-wave sleep and then into REM — where 79% of participants reported dreaming when woken, compared to only 7% when woken from NREM. The depth and character of your sleep literally changes on a 90-minute clock all night long.

This is why waking up in the middle of a deep cycle feels so disorienting, and why some people wake naturally at the end of a cycle feeling alert even after fewer total hours. You're not just sleeping — you're cycling.

Infradian Rhythms: The Long Game

Infradian rhythms are the ones that operate below the threshold of daily awareness. The menstrual cycle is the most documented: an approximately 28-day biological program controlled by cascading hormones across follicular, ovulatory, and luteal phases.

But as Dr. Erin Higgins, OB/GYN at the Cleveland Clinic, notes: "Very few people have a perfect 28-day cycle every single month." Metabolism, diet, stress, sleep disruption, body weight — all of these factors feed into the infradian system. High cortisol from sustained stress can cause cycles to become irregular or stop entirely. This isn't a malfunction; it's the body's timing system responding to signals from the environment, exactly as it was designed to do.

Seasonal affective disorder operates on an even longer infradian scale — a mood and energy pattern that follows the light cycle of the year. The fact that SAD qualifies as an infradian rhythm means it's not just "feeling a bit down in winter." It's a biological rhythm operating on a months-long cycle, driven by the same light-sensitive machinery that governs every other clock in your body.

Light Controls All Rhythm Types Simultaneously

The most striking demonstration of how deeply interconnected these timing systems are comes from a 1967 experiment by Reinberg. A young woman spent three months in a cave with only the dim light of a miner's lamp. Her daily cycle lengthened slightly — her day became 24.6 hours. But her menstrual cycle also changed: it shortened from 28 days to 25.7 days. And crucially, after she emerged, it took over 12 months before her menstrual cycle returned to normal.

This single study reveals something important: light doesn't just govern your sleep. It governs the entire architecture of your biological time. Circadian rhythms and infradian rhythms are not independent systems; they share the same foundational synchronization mechanism. Change the light, and you change everything downstream.

When Rhythms Overlap: Cortisol and the Menstrual Cycle

Some physiological systems are governed by more than one timing layer at once, and cortisol is the clearest example. Cortisol release has a classic circadian pattern — it peaks in the early morning and declines through the day. But it also pulses in ultradian bursts throughout the day, roughly every 60–90 minutes, independent of the daily arc. Cortisol operates on both a 24-hour clock and a sub-hourly clock at the same time.

The menstrual cycle shows similar complexity. Women with ovulatory cycles have a circadian rhythm superimposed on the longer infradian rhythm — and the two interact. Research on women in different menstrual phases shows that the follicular and luteal phases affect the regulation of REM sleep differently. Reproductive hormones don't just regulate reproduction; they influence circadian rhythm and sleep architecture. The rhythms aren't separate departments — they're the same organization, running on different schedules.

Social Cues Can Synchronize Biological Rhythms

McClintock observed something unusual in 1971: women living in the same college dormitory tended to have their menstrual cycles converge over time. Follow-up research by McClintock and Stern in 1988 tested this more rigorously, exposing 29 women to pheromone samples from other women at specific points in the cycle. The result: 68% of women's cycles shifted to synchronize with the pheromone donor.

What this means is that biological rhythms aren't just internal and light-driven — they can also be entrained by social chemical signals. The pheromones acted as exogenous zeitgebers for an infradian rhythm. Your body's timing is, in a real sense, not entirely private. The people around you can influence your biological clock.

Phase Misalignment: When Your Clocks Disagree

The modern world has a rhythm problem. Artificial light extends our days. Screens emit blue light at midnight that the SCN interprets as morning. Shift workers flip their entire circadian architecture on a rotating schedule. Frequent travelers impose jet lag repeatedly. The result is chronic phase misalignment — a state where your inherited clock period and your lived environment are pulling in different directions.

The consequences are not just feeling groggy. Phase misalignment is associated with acute cognitive decline, metabolic disruption, and broader health effects. Researchers have flagged it as a public health concern — because it's not an individual quirk but a systemic feature of contemporary life for millions of people working or living on schedules that don't match their biology.

The reason chronotype matters so much is precisely this: people inherit different natural clock periods, and a one-size-fits-all schedule creates phase misalignment for large portions of the population by default.

How Long Does It Take to See a Pattern?

One of the underappreciated aspects of infradian rhythm tracking is the timescale required to make sense of anything. A single irregular cycle tells you almost nothing. As Dr. Higgins puts it:

This is true beyond the menstrual cycle. Mood patterns, energy fluctuations, sleep quality, stress responses — any system running on an infradian timescale requires months of data before the signal becomes visible above the noise. Single-cycle anomalies mean nothing; sustained patterns across three to six months reveal truth.

This is one of the hardest things about self-knowledge: the timescale of meaningful patterns is longer than the timescale of our attention. We notice one bad week and panic. We miss the slow seasonal shift that's been affecting us for three years.

What This Means For You

Your body is not a single machine running one program. It's a nested system of timekeepers, each cycling on its own schedule, all synchronized — in ideal conditions — by the same external cues: primarily light, but also social signals, meal timing, and activity patterns.

Understanding cyclical patterns in human biology doesn't require a lab. It requires paying attention over time. A few things the science makes clear:

• Light is your most powerful biological lever. Morning light anchors your master clock. Evening artificial light delays it. The simplest biological optimization available to most people is getting outside in the morning.
• Your organs have their own timing. The liver's 12-hour metabolic rush hours mean that when you eat matters almost as much as what you eat.
• Short-term anomalies are noise. Infradian patterns — whether mood, energy, hormonal, or seasonal — require weeks to months of observation before they become legible.
• Phase misalignment is real, not just "tired." If your schedule consistently fights your biology, the effects compound over time. It's not a personal failing — it's a timing mismatch with measurable consequences.

The cave explorer who emerged 28 days displaced from reality wasn't doing anything wrong. His body was running exactly as designed. It just needed light — an anchor back to the shared rhythm of the world. Most of us do too.

Why This Is What zMotif Was Built For

The core insight from chronobiology is that patterns only become visible at the right timescale. A single bad day tells you nothing. A single great week might be random. But three months of daily check-ins — mood, energy, who you were with, what you did — starts to reveal the biological rhythms running beneath your conscious awareness.

This is exactly how zMotif's Smart Pattern Engine works. It doesn't surface an insight after one data point. It waits until the pattern is statistically significant — the same principle Dr. Higgins describes when she says tracking over three to six months gives you the bigger picture. zMotif's zMotif Calendar operates across multiple timescales simultaneously, just like your body does: daily snapshots that become weekly stories that become monthly and seasonal patterns.

And the longer-than-daily rhythms — the infradian cycles that affect your mood, energy, and social bandwidth on schedules you can't consciously perceive — are precisely what zMotif's Mystery Layer is designed to surface. You don't have to live in a cave to discover your biological rhythms. You just have to pay attention, consistently, and let the patterns emerge.

Sources: Troscriptions (Matthew Lees, PhD, reviewed by Scott Sherr, MD); Psychology Hub — Biological Rhythms: Circadian, Infradian and Ultradian; Cleveland Clinic (Dr. Erin Higgins, MD)