Biorhythms

That is the whole idea behind biorhythms. One wave for the body, one for the feelings and one for the mind, each with its own length, each rising and falling on its own timetable from your first breath. On any day of your life you are sitting somewhere on all three.

I include them in the Library because people ask about them, and because they are a lovely way to start noticing your own rhythms. I will also be straight with you about what the research says. You deserve both halves.

The idea goes back to Wilhelm Fliess, a Berlin doctor and friend of Sigmund Freud, who proposed the 23 and 28 day cycles in the late 1800s. The 33 day cycle was added in the 1920s, and biorhythm charts had their big moment in the 1970s.

The three cycles

The physical cycle runs for 23 days. It covers energy, strength, coordination and how quickly you recover. In its high half the body feels capable, and it is the natural time for demanding physical work or hard training. In its low half the body is asking for rest and repair. Nothing is wrong with you. The body is doing its maintenance, and walking, stretching and an early night serve you better than pushing through.

The emotional cycle runs for 28 days, close to the length of a lunar month. It covers mood, resilience and patience with other people. In the high half you bounce back quickly and can give without keeping score, so it is a good time for the important conversation. In the low half you are more easily hurt and more inclined to want your own company. Journal, be kind to yourself, and let a big relationship decision wait if it can.

The intellectual cycle runs for 33 days, the longest of the three. It covers clarity, memory, concentration and how well the right words arrive. In the high half learning sticks and careful decisions come more easily. In the low half you may read the same paragraph three times. Routine tasks go fine, and creative work that does not depend on strict logic often flows well.

How they are worked out

Everything starts with how many days you have been alive. Count from your date of birth to the day you want to look at, leap days included. Divide that total by the length of each cycle. The whole number is how many complete cycles you have lived through, and the remainder is the day of the current cycle you are on. Day 0 is the first day of a fresh cycle.

Each cycle is drawn as a smooth wave, the kind mathematicians call a sine curve. It starts on the middle line on the day you are born, rises to its peak a quarter of the way through, comes back to the middle line at the halfway point, sinks to its lowest at three quarters, and returns to the middle line as the next cycle begins. The first half of every cycle is the high phase and the second half is the low phase.

The days that get the most attention are the critical days, when a wave crosses the middle line and switches from high to low or back again. Each cycle has two: its first day and its halfway point. For the physical cycle that is day 0 and the crossing between days 11 and 12. For the emotional cycle it is day 0 and day 14. For the intellectual cycle it is day 0 and the crossing between days 16 and 17. Tradition says these are the days you are least steady in that part of yourself, so the old advice is simple. Stay off the ladder, leave the charged conversation for another day, and check your sums twice.

A worked example

Sarah Jane Mitchell is an invented person, born on 14 March 1987. Let us read her for Friday 16 October 2026.

From 14 March 1987 to 14 March 2026 is 39 years, and 39 times 365 is 14,235. Ten leap days fall in that stretch (1988 and every fourth year through to 2024), which makes 14,245. From 14 March to 16 October 2026 is another 216 days. Her total is 14,461 days.

Physical: 14,461 divided by 23 is 628 cycles with 17 left over. Day 17 of 23 puts her right at the bottom of the low half.

Emotional: 14,461 divided by 28 is 516 cycles with 13 left over. Day 13 of 28 is the very end of the high half, so tomorrow, day 14, is an emotional critical day.

Intellectual: 14,461 divided by 33 is 438 cycles with 7 left over. Day 7 of 33 is close to the peak, which arrives just after day 8.

Put together, her body wants rest, her mind is sharp and her feelings are about to turn. A good day for desk work, a poor day for heaving pot plants around the garden, and tomorrow a day to go gently in conversation. Her next physical critical day is Thursday 22 October, when a new 23 day cycle begins.

What the research says

Now the part I owe you. Biorhythms have been tested many times. Through the 1970s and 1980s researchers set people's critical days against accident records, sporting results and work performance, and found nothing beyond what chance would give. Later reviews came to the same conclusion. The body does have real rhythms, sleep and hormones among them, but none has been shown to run on a fixed count of 23, 28 or 33 days from birth.

So I offer biorhythms as a reflection tool, not a forecast. Please do not move a medical appointment or change how you drive because a chart says it is a critical day.

What they are good for is noticing. Work out your three cycles for the coming month, and each evening jot down a word or two about your energy, your mood and how clear your head felt. At the end of the month, lay your notes beside the waves. If they line up, that is interesting. If they do not, you have still spent a month paying close attention to your own patterns, and that is the real prize. You will be your own teacher.

Your Personal Year, Month and Day, which come from the same birth date, describe the theme of the season you are in. Biorhythms, at best, describe the weather in your body, heart and head while you meet it. The Personal Year post in the daily posts shows you how to find yours.

Which of the three waves do you think you would feel most clearly?

If you would like your own numbers read as part of a full chart, my readings are here.