Biorhythm Calculator
Track your 23-day Physical, 28-day Emotional, and 33-day Intellectual cycles from birth — plus four extended Thommen 1973 cycles.
Photo by Benjamin Voros on Unsplash
What Is Biorhythm?
In 1906 the Berlin physician Wilhelm Fliess proposed that the human body runs on fixed-period cycles starting from birth. His original three were the Physical cycle (23 days — strength, endurance, coordination), the Emotional cycle (28 days — mood, sensitivity, creativity) and the Intellectual cycle (33 days — focus, memory, reasoning). Each is modeled as a pure sine wave swinging between +100% peak and -100% low.
The Extended Cycles
George Thommen popularized biorhythm in his 1973 book Is This Your Day? and added four longer cycles: Intuitive (38d), Aesthetic (43d), Self-awareness (48d), and Spiritual (53d). These are not part of Fliess’s original theory — Thommen layered them on top of the classical three. We surface them because the fuller picture often surprises people who only know the three.
Honest framing
Biorhythm has been tested multiple times since the 1970s and no statistically significant predictive validity has been found. The math is exact; the predictive claim is folklore-grade. Use this as a structured self-reflection prompt — a way to notice when you feel high or low — not as a diagnosis or a planning tool for real-world consequences.
Biorhythm FAQ
What is a critical day?
A critical day is when one of the biorhythm curves crosses zero — the transition between positive and negative. In folklore framing these are the days when you are most variable in that domain. A triple-critical day — all three classical cycles crossing zero on the same day — happens roughly once every 7-9 years.
Why doesn’t birth time matter?
In biorhythm theory the cycles tick by full days, anchored on midnight of your birth date. CelestKin’s in-app version adds hour-precision tracking (using your birth time) but the difference within a single day is small.
What can I actually use this for?
Self-reflection. The cycle math gives you a structured prompt — “why might I be feeling more tired this week?” — without claiming to predict events. It pairs well with a journaling habit. We do not recommend using it to schedule surgery, sports performance, or any real-world consequential decision.
Try More Free Calculators
Explore your charts across multiple astrological traditions