Physical cycle
The physical cycle is the shortest of the three biorhythm cycles: it lasts 23 days and starts on the day of birth. The classic method links it to energy, stamina and drive. It spends eleven days in the high phase and eleven in the low one, with a critical day at each crossing between the two.
It is the cycle Wilhelm Fliess described first, in 1897, and called the male cycle. In the classic method it governs the most concrete side of a day: the strength available, endurance under effort, the readiness of reflexes.
With a 23-day period it is also the cycle that changes phase most often: a critical day roughly every eleven and a half days, which is about thirty-two a year, against some twenty-six for the emotional cycle and twenty-two for the intellectual one. Of the three, it is the one people tend to notice first when they compare the chart with how they feel.
In the couple comparison it has a property you can check with a calendar. Two people both have a 23-day physical cycle, so the distance between their positions never changes: if the two birth dates are five days apart, the two cycles stay five days out of step for life. The widest offset is about eleven and a half days, half a cycle, and then the two sit in opposite phases. A couple's physical axis is therefore a trait of the couple, not of a single day.
For someone born on 15 June 1990, on 2 September 2026 the physical cycle is on day 4 of 23: in the first half of the cycle, so in the high phase.