numerologyBy DailyDestiny Editorial Team

Biorhythms: The Three Cycles and Whether They Hold Up

Biorhythms are three fixed sine waves said to start at birth and run for life: a 23-day physical cycle, a 28-day emotional cycle and a 33-day intellectual cycle. This guide covers where the biorhythm cycles came from, how to calculate a biorhythm chart by hand, what a critical day means, and what researchers found when they tested the theory.

What are biorhythms and what do the three biorhythm cycles track?

Biorhythms are three fixed-length cycles that, in this tradition, begin at the moment of birth and run unchanged until death: a 23-day physical cycle, a 28-day emotional or sensitivity cycle, and a 33-day intellectual cycle. Each biorhythm cycle is drawn as a sine wave starting at zero on the birth date, rising into a positive half, falling back through zero, dropping into a negative half, and returning to the start. The physical cycle is said to govern stamina, coordination, resistance to illness and bodily confidence. The emotional cycle covers mood, sensitivity, creative responsiveness and how easily a person is irritated or moved. The intellectual cycle covers concentration, memory, reasoning and the appetite for detailed work. Nothing in the theory depends on the hour of birth, on sex, on location or on any birth chart. The only input is the number of days a person has been alive, which means two people born on the same calendar date carry an identical biorhythm chart forever. That property separates biorhythms from astrology, and it is what made the theory attractive to the engineers and office managers who spread it: arithmetic anybody can run with a calendar.

The vocabulary attached to the three biorhythm cycles came from the popular books of the 1960s and 1970s rather than from the original German-language sources. The positive half of a curve is the discharge phase or high phase, described as the stretch where the body or mind spends energy it has banked. The negative half is the recharge phase or low phase, and biorhythm writing is careful not to call it a bad phase: a low physical reading is when the body restores itself, and pushing hard through it is what practitioners warn against. Somebody whose physical curve sits at plus 90 percent while the intellectual curve sits at minus 80 percent is not having a good day or a bad day; the reading is that the legs are willing and the proofreading will be sloppy. Because the three biorhythm cycles have different lengths they drift in and out of alignment continuously, so combinations rarely repeat. All three return to zero together only after 23 x 28 x 33 = 21,252 days, which is 58 years and roughly 68 days, the point biorhythm literature calls the biorhythmic birthday.

Which of the biorhythm cycles is the shortest?

The 23-day physical cycle is the shortest of the biorhythm cycles, so it swings fastest: it crosses the zero line roughly every 11.5 days and completes almost 16 full turns a year. The 33-day intellectual cycle is the longest and moves most slowly, completing about 11 turns a year and holding each phase for over eight days.

Do biorhythms use your time of birth?

No, biorhythms ignore time of birth entirely and count only whole days since the birth date. A baby born at 1 a.m. and one born at 11 p.m. on the same date carry identical biorhythms for life. This is a deliberate feature of the theory, not an oversight, and it is the sharpest structural difference between a biorhythm chart and an astrological one.

What are the extra cycles some biorhythm calculators add?

Some biorhythm calculator programs add three later inventions to the original set: a 38-day intuitive cycle, a 43-day aesthetic cycle and a 48-day awareness cycle. None of them appear in the early Fliess or Swoboda material. They were attached during the 1970s biorhythm boom, and which ones a program shows varies from one piece of software to the next.

Who invented biorhythms, and where did the 23, 28 and 33-day periods come from?

Biorhythms began with Wilhelm Fliess, a Berlin ear, nose and throat specialist and the close correspondent of Sigmund Freud through the 1890s. Fliess proposed that human life is governed by two periods, 23 days and 28 days, which he labeled male and female and held that every person carries in some mixture. He gathered birth, illness and death dates from his practice and family records, and set out the system in Der Ablauf des Lebens in 1906. Hermann Swoboda, a psychologist at the University of Vienna, published on the same two periods at almost the same time, tracing recurrences of dreams, moods and fevers at 23 and 28-day intervals. Which of the two men had the idea first became a bitter dispute, and it helped end the Freud-Fliess friendship. The 33-day intellectual cycle belongs to neither of them. Biorhythm literature credits it to Alfred Teltscher, described as an engineering instructor in Innsbruck who is said to have derived it from student examination results in the 1920s, giving the theory the third curve that every later biorhythm chart shows.

Biorhythms reached an English-speaking mass audience through George S. Thommen, a Swiss-born businessman in New York who imported the idea from Central Europe, sold charting kits, and published Is This Your Day? in 1964. Thommen supplied the three-curve layout and the discharge and recharge language that every later biorhythm calculator inherited. The boom came a decade afterward, when Bernard Gittelson's Biorhythm: A Personal Science in 1975 and a wave of syndicated newspaper columns put charts in front of readers who had no interest at all in Fliess's arithmetic. Consumer electronics finished the job. By the late 1970s manufacturers including Casio were selling pocket devices whose only function was to print a biorhythm chart from a birth date, and biorhythm modes turned up on ordinary calculators and early digital watches. Employers experimented too: transport operators and insurers in Japan, Switzerland and the United States are widely reported to have tried warning programs built on employees' critical days, and their internal accident tallies became the case studies quoted throughout the popular books. Those tallies were never gathered as controlled trials, which is the weakness researchers took up in the years that followed.

What was Wilhelm Fliess's medical theory behind biorhythms?

Fliess taught a doctrine of nasal reflex neurosis, holding that the nose was connected to the genitals and to sexual function, and treating patients with nasal surgery and applications of cocaine. His 23 and 28-day periods came out of that same framework of male and female substances. The notorious case is his operation on Emma Eckstein, during which surgical gauze was left packed in her nasal cavity and caused a near-fatal hemorrhage.

Why did Freud and Fliess fall out over biorhythms?

Freud had discussed Fliess's unpublished work on the 23 and 28-day periods with Hermann Swoboda. When Swoboda published on the same cycles, Fliess accused Freud of passing his material along, and the friendship broke down in the early 1900s. Freud's surviving letters to Fliess are the primary source for the whole episode and for how seriously Freud once took the periods.

Who added the 33-day intellectual cycle to biorhythms?

The 33-day intellectual biorhythm cycle is credited to Alfred Teltscher, said to have been an engineering instructor in Innsbruck who studied his students' examination results in the 1920s. Unlike Fliess and Swoboda, whose books can be found in libraries, Teltscher left almost no traceable publication, so the attribution rests on repetition through the popular biorhythm literature rather than on a citable source.

How do you calculate a biorhythm chart without a biorhythm calculator?

Calculating a biorhythm chart by hand takes two steps. First, count the total days elapsed since the birth date, treating the day of birth as day zero. Second, feed that count into the formula value = sin(2 x pi x days / period) x 100, running it three times with the period set to 23 for physical, 28 for emotional and 33 for intellectual. The result is a percentage between minus 100 and plus 100. Take a birth date of June 4, 1988 charted for August 16, 2026. Thirty-eight full years is 13,870 days, plus nine leap days from 1992 through 2024, plus the 73 days from June 4 to August 16, giving 13,952 days. Dividing by each period leaves the position in the cycle: 13,952 divided by 23 leaves 14, by 28 leaves 8, and by 33 leaves 26. Those positions produce a physical reading of minus 63 percent, an emotional reading of plus 98 percent and an intellectual reading of minus 97 percent. Every biorhythm calculator on the web runs this same arithmetic behind a nicer interface.

Before the sine formula was easy to evaluate, biorhythm practitioners worked with mechanical aids. Swoboda produced a slide rule for locating recurrences at 23 and 28-day intervals, and Thommen sold a charting kit with printed grids and plastic curve templates the user slid along a dated baseline to trace the month ahead. The remainder is what those tools were really finding: once you know the position within each biorhythm cycle you know the phase, and the exact percentage matters less than whether the curve sits above or below the axis. To plot a month, note the remainder on the first and step forward day by day, wrapping each count back to zero at 23, 28 or 33. Most disagreements between two biorhythm calculators come from one of two places: whether the birth day counts as day zero or day one, which shifts every curve by a full day, and how leap days and local midnight are handled when birth date and chart date sit in different time zones.

What is the biorhythm formula?

The biorhythm formula is value = sin(2 x pi x t / P) x 100, where t is the number of days lived and P is the cycle length: 23 for physical, 28 for emotional and 33 for intellectual. The output runs from minus 100 to plus 100. A peak falls a quarter of the way through the cycle and a trough three quarters of the way through.

How do you count days alive for a biorhythm chart?

Multiply the completed years by 365, add one for each February 29 that fell inside the span, then add the days from the birthday to the chart date in the final year. The day of birth counts as day zero, so a person is on day one of their biorhythms on the day after they were born. This is the single most common source of error in hand-drawn biorhythm charts.

Why do two biorhythm calculators give different numbers?

Two biorhythm calculators disagree for three reasons: an off-by-one on whether the birth day is day zero or day one, differing treatment of leap days in the day count, and time zone handling that shifts the chart date across local midnight. A single day of drift moves the physical curve by roughly 16 percentage points, which is enough to change a reported phase near a crossing.

What is a critical day on a biorhythm chart?

A critical day on a biorhythm chart is a day when one of the three curves crosses the zero line, switching from the positive half of the cycle to the negative half or back again. Biorhythm practitioners do not read a critical day as unlucky. The reading is that the system is changing state rather than resting at a settled level, in the same way a tide is least predictable at the turn. Each biorhythm cycle produces two crossings per period, at the start and at the midpoint. The physical cycle crosses on days 0 and 11.5 of every 23, the emotional cycle on days 0 and 14 of every 28, and the intellectual cycle on days 0 and 16.5 of every 33. Because 23 and 33 are odd, the physical and intellectual crossings fall mid-day and straddle two calendar dates, while the 28-day emotional crossings land cleanly on a day boundary. When two curves cross on the same date, biorhythm writing calls it a double critical day; when all three do, a triple critical, which is the rarest configuration a biorhythm chart produces.

The arithmetic of critical days is worth doing, because it settles how much a critical-day warning is actually saying. The physical cycle delivers about 31.8 critical days a year, the emotional cycle about 26.1 and the intellectual cycle about 22.1, for roughly 80 crossings a year in total. Since crossings from different cycles sometimes land on the same date, the count of calendar days carrying at least one crossing comes to about 20 percent of the year, one day in five. Double criticals arrive around five or six times a year. Triple criticals, where all three curves cross within the same calendar day, occur only about eight times in the full 21,252-day span, roughly once every seven years. This is why the practical advice in the 1970s books focused on double and triple criticals rather than single ones. It is also why the theory is hard to test kindly: a system that flags a fifth of all days as unstable will collect a great many apparent hits by arithmetic alone.

How many biorhythm critical days are there in a year?

There are about 80 biorhythm critical crossings a year across the three cycles, roughly 32 physical, 26 emotional and 22 intellectual. Because some fall on the same date, about 20 percent of calendar days carry at least one crossing. Double critical days, when two of the biorhythm cycles cross together, occur about five or six times a year.

Is a critical day the same as a low biorhythm day?

No. A critical day is the moment a biorhythm curve passes through zero, while a low day is when the curve sits deep in its negative half. Biorhythm practitioners treat these differently: a low physical reading calls for rest and recovery, whereas a physical critical day calls for care with anything requiring precise coordination, regardless of how the other curves read.

How rare is a triple critical day in biorhythms?

Triple critical days, when all three biorhythm cycles cross zero within the same calendar day, happen about eight times over the full 21,252-day period, or roughly once every seven years. Because the 23, 28 and 33-day cycles share no common factor beyond one, their crossings drift apart continuously and only occasionally coincide. This rarity is exactly why the popular literature made so much of them.

How do you read a biorhythm chart across a month?

A biorhythm chart is read as three labeled sine waves plotted over a dated horizontal axis, usually one month wide, with a vertical scale running from minus 100 at the bottom through zero in the middle to plus 100 at the top. The physical, emotional and intellectual curves are drawn in different colors or line styles, and readers scan for three features: which side of the zero line each curve sits on, where the peaks and troughs fall, and which dates carry crossings. In this tradition the sign matters more than the exact percentage. A physical curve at plus 40 and one at plus 90 are both read as a high physical phase, and the distinction most readers act on is high versus low rather than 40 versus 90. The practical use is scheduling rather than prediction. A month showing an intellectual high in the first ten days and a physical high in the last ten is read as an argument for putting the contract review early and the long training runs late. A biorhythm chart never says what will happen on a date; in the tradition it describes capacity only.

Regular users of a biorhythm chart tend to develop a habit with more merit than the theory it rests on: they keep a short daily log of energy, mood and focus, then compare it against the chart at the end of the month. Done with the log written before the chart is consulted, this is a self-experiment, and it will tell a person more about their own patterns than the sine waves will. Done the other way around, it becomes an exercise in noticing whatever the chart predicted, which is how a great many people came away from the 1970s biorhythm boom convinced. Where a biorhythm chart earns its keep for its users is as a scheduling prompt, in the way a coin flip earns its keep: it forces a decision about the month to be made deliberately and in advance rather than by drift. The person who blocks out demanding work for one half of the month and recovery for the other has planned a month, whatever the 23 and 33-day arithmetic is doing.

What does it mean when a biorhythm cycle reads plus 100?

A reading of plus 100 means that biorhythm cycle is at its peak, a quarter of the way through its period: day 5.75 of the 23-day physical cycle, day 7 of the 28-day emotional cycle, or day 8.25 of the 33-day intellectual cycle. Practitioners read peaks as the settled top of a phase rather than as a spike, since the curve moves slowly near its maximum.

What is biorhythm compatibility?

Biorhythm compatibility compares two people's charts and scores how closely their curves track one another, cycle by cycle. The score depends entirely on the gap between the two birth dates. A gap that is a multiple of 23 puts both physical curves in lockstep, a multiple of 28 does the same for the emotional curves, and a gap of half a period puts the two curves in exact opposition.

Can a biorhythm chart predict a specific event?

No. Even within the tradition, a biorhythm chart describes a person's state rather than external events, so it makes no claim about outcomes such as a job offer, an accident or a test result. The strongest claim biorhythm practitioners make is that a critical day is a poor time to rely on precision, which is a statement about capacity, not about what the day will bring.

Do biorhythms work when tested against real data?

No, biorhythms do not hold up under controlled testing. The claims that made the theory famous in the 1970s came from retrospective work: an investigator would take a file of accidents, calculate the charts of the people involved, and report how many fell on critical days, without establishing what proportion should be expected by chance. When researchers ran the comparison properly, the effect disappeared. Aeromedical teams in the late 1970s matched several thousand general aviation accident records against the pilots' biorhythm charts and found the distribution of critical days indistinguishable from the base rate. Similar work on industrial injuries, traffic accidents, athletic performance and examination scores returned the same answer. Terence Hines reviewed the accumulated biorhythm literature in Psychological Reports in 1998 and concluded that the theory has no valid support, and that the studies appearing to favor it shared identifiable methodological faults. The figure to hold in mind when reading any positive biorhythm claim is the base rate: about 20 percent of all days carry a crossing, so a report that a fifth of accidents fell on critical days is a report of nothing at all.

The corporate reports that circulated during the biorhythm boom are worth taking seriously as a puzzle, because several of them did describe genuine drops in accident rates. The explanation does not require the 23, 28 and 33-day cycles to be real. A firm that adopts a biorhythm program starts telling drivers and machine operators, on roughly 80 mornings a year, that today is a day for extra care. It also starts counting accidents attentively, often for the first time, and it signals that management is watching safety. Any of those would move the numbers, and none is a test of biorhythms. Nothing in those programs isolated the cycles from the warning, which is what a controlled trial would have to do: flag a randomly chosen 20 percent of days for one group and the genuine critical days for another, keeping everybody blind to which is which. The studies that came closest to that design produced no difference, and the corporate reports with the dramatic numbers never attempted it.

Has any study supported biorhythm theory?

Individual papers reporting positive results for biorhythms have appeared, mostly in the 1970s, but they have not survived scrutiny. Reviews of the field, including Terence Hines's 1998 assessment in Psychological Reports, found that the supportive studies were retrospective, used inappropriate comparison figures, or failed to specify the critical-day definition before the analysis. Attempts to replicate them under stricter conditions returned chance-level results.

Why did 1970s companies report fewer accidents after adopting biorhythms?

Companies that adopted biorhythm programs changed several things at once: they warned employees on about 80 days a year, they began recording accidents more carefully, and they made safety visibly a management priority. Any of these can reduce reported accidents on its own. Because none of the programs separated the warning from the underlying biorhythm cycles, the figures cannot show which factor was responsible.

Is there a real 23-day physical rhythm in humans?

Chronobiology has not identified any 23-day physical rhythm in humans. Measured human rhythms run on other scales entirely: the roughly 24-hour circadian cycle, ultradian cycles of around 90 minutes, and seasonal patterns. Nothing in the physiological literature corresponds to a fixed 23-day period that is identical in every person and locked to the date of birth.

What are the strongest objections to biorhythm theory?

Five objections to biorhythm theory are structural rather than statistical, meaning they apply before any data is collected. First, Fliess built his system on the claim that any interval can be expressed as a combination of 23s and 28s, which is mathematically true for every whole number and therefore explains everything and forbids nothing. Second, the 28-day emotional cycle is exactly four weeks, so a person's emotional critical days permanently fall on the same weekday they were born on, a pattern that would be trivially visible in weekday accident statistics and is not. Third, the periods are held to be identical in every human being, whereas every rhythm physiology has actually measured varies between individuals and within one individual over time. Fourth, roughly 20 percent of days carry a crossing, a hit rate generous enough to sustain the theory on coincidence alone. Fifth, birth is a strange zero point for cycles governing physiology that is plainly running months before delivery, and modern induction and scheduled Caesarean sections mean the supposed starting date of the biorhythm cycles can be moved by an appointment.

The first objection deserves the arithmetic, because it is the cleanest single result about biorhythms. Fliess argued that the intervals between births, illnesses and deaths in his records could be written as combinations of his two periods, and he produced pages of such calculations. But 23 and 28 have no common factor beyond one, and any two whole numbers in that relationship can be combined with positive and negative multipliers to reach every whole number there is. To take an example, 1 = (11 x 23) - (9 x 28), and once you can reach 1 you can reach anything. Fliess's method therefore had no possible failing case: whatever interval his records threw up, a combination existed, and finding it counted as confirmation. A theory that no observation can contradict is not making a claim about the world, and critics noticed this during Fliess's own lifetime. The later three-cycle biorhythm chart dropped that arithmetic in favor of fixed sine waves, which at least can be tested, and testing is where it failed.

Why do emotional biorhythm critical days always fall on the same weekday?

The emotional biorhythm cycle is 28 days, which is exactly four weeks, and its two crossings fall 14 days apart. Since both 28 and 14 divide evenly by 7, every emotional critical day in a person's life falls on the same weekday as their birth. Someone born on a Saturday still has Saturday emotional criticals at age 80. The 23 and 33-day cycles drift instead.

What percentage of days are biorhythm critical days?

About 20 percent of calendar days carry at least one biorhythm crossing, which is one day in five. The three cycles generate around 80 crossings a year between them, and some share a date. Any claim that a given percentage of accidents fell on critical days has to be compared against this 20 percent figure to mean anything at all.

Does a scheduled Caesarean change your biorhythm chart?

Yes, by the theory's own logic. Because biorhythm cycles are held to start on the date of delivery, moving a birth by a day through induction or a scheduled Caesarean shifts all three curves by a day for the rest of that person's life. Biorhythm theory offers no account of why a clinical appointment would set a lifelong physiological clock.

How do biorhythms differ from circadian rhythms and real chronobiology?

Biorhythms and circadian rhythms are unrelated, and confusing them is the most common mistake made about the theory. Circadian rhythms are roughly 24-hour cycles that have been measured directly, traced to their molecular machinery, and shown to be reset by light. Franz Halberg coined the term circadian in the late 1950s, and the 2017 Nobel Prize in Physiology or Medicine went to Jeffrey C. Hall, Michael Rosbash and Michael W. Young for working out the genetic clockwork behind them. Chronobiology also describes ultradian rhythms shorter than a day, including the basic rest-activity cycle of roughly 90 minutes that Nathaniel Kleitman identified in sleep research. Every measured rhythm varies from person to person and shifts with age, light exposure, shift work and travel. Biorhythm cycles are the opposite on all three counts: postulated rather than measured, identical in every person, and fixed to the birth date rather than entrained by anything in the environment.

The 28-day emotional cycle is where the confusion usually starts, because the menstrual cycle averages close to 28 days and Fliess drew his female period from exactly that observation. The comparison breaks down as soon as it is examined. Menstrual cycle length varies widely between women and from month to month in the same woman, responds to illness, stress, weight and age, and ceases at menopause. The 28-day biorhythm cycle does none of that: it applies equally to men, never varies by a day, and continues unchanged to the end of life. For a reader who came to biorhythms wanting a way to schedule around personal energy, chronobiology offers the version that survives testing. Chronotype is measurable, alertness and body temperature follow a genuine daily curve, and reaction time is worst in the small hours. Keeping a two-week log of energy by hour is the honest form of the charting habit, and it produces a curve that belongs to the person who kept it.

Is the 28-day emotional biorhythm the menstrual cycle?

No. Fliess took the number 28 from the average menstrual cycle, but the 28-day emotional biorhythm applies to men as well as women, never varies in length, and runs unchanged throughout life. Real menstrual cycles vary between individuals and between months, respond to stress and illness, and cease at menopause. The shared number is where the resemblance ends.

What is the difference between a biorhythm chart and a circadian rhythm chart?

A biorhythm chart plots three fixed sine waves of 23, 28 and 33 days calculated from a birth date, identical for everybody born that day. A circadian chart plots measured quantities such as core body temperature, melatonin or alertness across roughly 24 hours, differs between individuals, and shifts with light exposure, sleep schedule and travel.

Is there any harm in using a biorhythm calculator?

The main risk in using a biorhythm calculator is deferring something that matters because the chart shows a critical day: postponing a medical appointment, a difficult conversation or a decision that will not improve with waiting. A secondary risk is using a low reading to excuse behavior. Used as a prompt to plan a month deliberately, a biorhythm calculator costs nothing.

Frequently Asked Questions

What are the three biorhythm cycles and how long is each one?

The three original biorhythm cycles are the physical cycle at 23 days, the emotional or sensitivity cycle at 28 days, and the intellectual cycle at 33 days. Each is drawn as a sine wave beginning at zero on the day of birth, rising to a peak a quarter of the way through, returning to zero at the halfway point, dipping to a trough, and closing the loop. Wilhelm Fliess proposed the 23 and 28-day periods in the 1890s; the 33-day intellectual cycle was added later and is usually credited to Alfred Teltscher.

How do I calculate my biorhythms by hand?

To calculate biorhythms by hand, count the total number of days you have been alive, treating the day of birth as day zero, then apply the formula value = sin(2 x pi x days / period) x 100 with period set to 23, 28 and 33. For a June 4, 1988 birth date charted on August 16, 2026, the count is 13,952 days, which gives a physical reading of minus 63 percent, an emotional reading of plus 98 percent and an intellectual reading of minus 97 percent. Any biorhythm calculator is running exactly this arithmetic.

What is a critical day in biorhythms?

A critical day in biorhythms is a day on which one of the three curves crosses the zero line, moving from the positive half of the cycle to the negative half or the other way around. Biorhythm practitioners treat critical days as unstable rather than unlucky, on the reasoning that the system is switching states rather than sitting at a settled level. The physical cycle produces about 32 critical days a year, the emotional cycle about 26 and the intellectual cycle about 22, so roughly one day in five carries at least one crossing.

Do biorhythms work according to scientific studies?

No. Controlled studies of biorhythms have not supported the theory. When researchers matched large sets of aviation and industrial accident records against the pilots' and workers' biorhythm charts and compared the result to the expected base rate, critical days turned up about as often as chance predicts. Terence Hines reviewed the biorhythm literature in Psychological Reports in 1998 and concluded the theory is not valid, noting that the reports which appeared to support it were retrospective and lacked controls. The 20 percent critical-day base rate is high enough that unsystematic hits are easy to accumulate.

Are biorhythms the same as astrology?

No. Biorhythms and astrology share nothing but a birth date. A biorhythm chart uses only the number of whole days elapsed since birth, ignoring time of birth, place of birth, planetary positions and sex, so everybody born on a given calendar date carries an identical chart for life. Astrology depends on birth time and location because the ascendant and house cusps change every few minutes. Biorhythm theory also makes no claim about personality or destiny, only about the daily state of the physical, emotional and intellectual cycles.

Can two people have the same biorhythm chart?

Yes. Any two people born on the same calendar date have identical biorhythm charts for the whole of their lives, since the only input is the number of days lived. Partial matches occur when birth dates are separated by a multiple of one of the biorhythm cycles: two people born 23 days apart run their physical curves in lockstep, and two born 28 days apart share the same emotional curve. Biorhythm compatibility scores in most software are built from exactly these offsets.

When do all three biorhythm cycles reset at once?

All three biorhythm cycles return to zero together after 21,252 days, the product of 23, 28 and 33, which works out to 58 years and about 68 days. Biorhythm writing calls this the biorhythmic birthday, the single day when the chart repeats the exact configuration it held at birth. Before that point the three cycles never fully realign, which is why a biorhythm chart shows so few repeated combinations. Calendar days on which all three curves cross zero at once occur about eight times across those 58 years.