Biorhythm Calculator (Daily Cycle Chart)
Time
Biorhythm theory, popularised in the late 19th and 20th centuries, models human well-being as three sine-wave cycles that start at birth: a 23-day physical cycle, a 28-day emotional cycle, and a 33-day intellectual cycle. Each cycle's value is sin(2π × daysSinceBirth / period) and ranges from −100% (low) to +100% (high). A 'critical day' is when a cycle crosses zero — supposedly a day of low energy or higher risk. This tool computes all three primary cycles for any (birth date, target date) pair, plus four secondary cycles often added in modern versions (intuition 38, aesthetic 43, awareness 48, spiritual 53). The chart shows ±15 days around the target so you can see what's coming. Hover the chart to read off any nearby day. Days since birth and the count to the next 1000-day milestone are reported alongside. **Important: biorhythm has no support in modern scientific research and is considered pop math, not medicine.** It's fun to look at and not harmful to know about — use it that way.
Hover the chart to inspect a specific day. Day 0 is the target date.
About biorhythm
Biorhythm models physical, emotional, and intellectual well-being as three sine waves with periods of 23, 28, and 33 days, all starting at 0% on the birth date. The value at any later day is sin(2π × daysSinceBirth / period), ranging from −100% to +100%. A critical day is when a cycle crosses zero. The theory has no empirical support — modern studies show no correlation between biorhythm phase and accidents, athletic performance, mood, or cognition. Treat the output as numerology, not science.
How to use
- Enter your birth date and a target date (defaults to today).
- Read off the current percentage for each cycle — green = high, gray = low, amber = critical (near zero crossing).
- Hover the chart to inspect nearby days. The dashed line marks the day under the cursor.
- Check the 'critical days' row to see which days in the ±15 window have a zero-crossing in any cycle.
- Toggle 'show secondary cycles' to add intuition, aesthetic, awareness, and spiritual cycles.
Frequently asked questions
- Is biorhythm a real scientific theory?
- No. Biorhythm theory was developed by Wilhelm Fliess in the 1890s and popularised by Bernard Gittelson in the 1970s, but controlled studies have consistently failed to find any predictive value. The American Psychological Association lists it as pseudoscience. Treat it as a fun visualisation, not medical advice or a planning tool.
- Where do the periods 23, 28, 33 come from?
- Fliess chose 23 and 28 days empirically (he claimed they were 'male' and 'female' biological cycles). Hans Schlieper later added 33 days as the 'intellectual' cycle. The secondary cycles (38, 43, 48, 53) were proposed in the 20th century. There's no biological mechanism known to produce sine waves of these specific periods — the periods are conventional, not measured.
- How is each value computed?
- For each cycle: value = sin(2π × daysSinceBirth / period). 'Days since birth' is the difference between the target date and birth date, in whole days (UTC midnight to UTC midnight). Cycles start at 0% on the birth date, rise to +100% at period/4, return to 0% at period/2, drop to −100% at 3·period/4, and complete the cycle at period days. The 'critical' label fires when consecutive day values straddle zero.
- What's a 'milestone day'?
- Every 1000th day since birth — your 1000-day, 10000-day, 20000-day milestones, etc. The 10000-day mark falls roughly on your 27th birthday + 138 days; the 30000-day mark around age 82. Some people celebrate these as decimal-system birthdays. The tool shows the next milestone and the remaining days.
- Why does my partner's chart look similar?
- Because the cycles are deterministic from birth date, two people born close in time will have nearly identical charts. Two people born exactly the same day will have identical charts forever. This is one of the simpler reasons biorhythm doesn't track real outcomes — actual mood and energy obviously don't synchronise with your birthday neighbours.
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