Biorhythm
A biorhythm is a classic model in which three fixed-length cycles begin on the day you are born: physical at 23 days, emotional at 28, intellectual at 33. Each cycle alternates a high phase and a low one, and it is calculated from the date of birth alone, with no birth time or place.
The word joins bios, life, and rhythmos, rhythm. The idea dates back to the late nineteenth century, when Wilhelm Fliess and Hermann Swoboda observed 23- and 28-day periodicities in their patients; the 33-day cycle was added in the 1920s by Alfred Teltscher. Modern chronobiology later confirmed that the body runs on cycles, and the study of internal biological clocks won the Nobel Prize in Medicine in 2017: that work concerns the 24-hour circadian rhythm, a different phenomenon from the three cycles of 23, 28 and 33 days, which remain a classic model for reading personal time.
The calculation is arithmetic: count the days since birth and divide by 23, 28 and 33. The remainder of each division tells you where in the cycle you are today. It is the same sum for everyone, and anyone can redo it.
For someone born on 15 June 1990, 13,229 days have passed by 2 September 2026: the physical cycle is on day 4 of 23, the emotional on 13 of 28, the intellectual on 29 of 33.