Biorhythm

A 1970s idea, rendered in glowing curves.

Enter your birth date to chart your physical, emotional and intellectual cycles for today, purely for fun.

How it works

Biorhythm theory models three sine-wave cycles starting at birth: physical (23 days), emotional (28 days) and intellectual (33 days). The tool counts the days since your birth date and shows where each wave sits today, from -100% to +100%.

When it helps

It's a fun, retro way to chart your 'day' — the theory was popular in the 1970s and still makes an entertaining daily check-in.

The formula behind the curves

Each cycle is a sine wave that starts at zero on the day you were born. For any date, the tool counts the days since your birth and calculates sin(2π × days ÷ cycle length) for each cycle, then shows it as a percentage from −100% to +100%. The physical cycle repeats every 23 days, the emotional cycle every 28 days and the intellectual cycle every 33 days.

Because the three lengths share no common factor, all three curves only line up again after 23 × 28 × 33 = 21,252 days — a little over 58 years. That's why everyone's chart looks different, even for people born in the same month.

How to read the chart

Above the centre line is described as a 'high' phase for that area and below it a 'low' or recovery phase. The days when a curve crosses zero are called critical days in the theory. The chart shows a window around today so you can see which way each curve is heading, not just today's value.

A little history

The 23- and 28-day cycles were proposed around 1900 by the Berlin physician Wilhelm Fliess and the psychologist Hermann Swoboda, and the 33-day intellectual cycle was added later. Biorhythm calculators became a craze in the 1970s, appearing in magazines, pocket calculators and arcade machines. Later studies found no real predictive power, which is why we present it purely for fun.

Frequently asked questions

Is biorhythm backed by science?

No. Controlled studies haven't found biorhythms to predict anything. Treat it as entertainment.

What does a zero crossing mean?

In the theory, days where a cycle crosses zero are 'critical days', when that area is supposedly least stable.

Can I check a different date?

The chart shows the days around today, so you can look a little ahead and behind. The values follow the same formula for any date, since they only depend on the number of days since birth.

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