Biorhythms

The idea

Three sine-wave cycles are said to start at zero on the day of birth:

• Physical cycle (23 days) — vitality, stamina, resistance.
• Emotional cycle (28 days) — mood, sensitivity, intuition.
• Intellectual cycle (33 days) — concentration, memory, logic.

When a cycle is in its high phase, the corresponding ability is supposedly enhanced; in its low phase, diminished. Days when a curve crosses zero are called "critical days".

What the evidence says

Unlike astrology, biorhythm theory has been tested extensively since the 1970s — precisely because it makes checkable predictions about accidents and performance. The verdict is unambiguous: no controlled study has found any relationship between cycles counted from a birth date and accidents, mood or achievement. Scientific consensus classes it as a pseudoscience.

Where such tools retain value for people who use them, it is as a prompt to stop and consider their physical form, mood and concentration — not as a forecast.

What it can still be good for

You don't have to believe an instrument predicts anything to get something out of it. Many people use biorhythms as a prompt to check in regularly on their energy, their mood and their concentration — three questions rarely asked otherwise. It is a logbook disguised as a graph.

The risk lies elsewhere: calling off an important decision because a “critical day” falls on it, or dismissing real fatigue because the physical curve is high. A symptom that persists is a matter for a doctor, not a chart.

Frequently asked questions

Further reading

• International Astronomical Union — official definition of the constellations and the astronomical zodiac
• NASA Science — why the zodiac and the constellations no longer line up
• European Space Agency — resources on the sky and celestial objects

Links to official or reference sources. They open in a new tab.

Calculating the biorhythm compatibility of two people

The theory holds that three cycles begin at birth and run for life: a physical cycle of 23 days, an emotional cycle of 28 days and an intellectual cycle of 33 days. Compatibility between two people is then measured by how far their cycles are in phase: the closer the gap between the two dates of birth is to a whole number of cycles, the stronger the claimed agreement.

The calculator below applies that theory faithfully. It predicts nothing — the rest of the page explains why — but it is at least honest about what it does.

Why the figure means nothing

Three reasons, in order of seriousness. First, the result never changes: the gap between two dates of birth is fixed, so your “compatibility” is the same at twenty and at seventy, in the first month and at the divorce. A quantity that does not vary cannot explain something that does.

Second, the cycles have no biological basis. Chronobiology knows plenty of real rhythms — the 24-hour circadian rhythm, hormonal cycles — but they differ between people, shift with light and sleep, and do not last a fixed number of days counted from birth. The 23, 28 and 33-day periods were asserted in the late nineteenth century by Wilhelm Fliess, without measurement.

Third, the theory has been tested. Dozens of studies looked for a link between cycle position and industrial accidents, road accidents, athletic performance or mortality. The fullest review, by Terence Hines in 2004, found no effect. This is not an open question; it is a closed one.

So why does this page exist

Because the word circulates, the calculation is entertaining, and it is better found here with the explanation than elsewhere without it. If you are looking for what actually predicts whether a couple lasts, the answer is not in dates of birth: it is in how two people talk to each other when they disagree.