The biorhythm theory was developed by Wilheim Fliess in the late 19th century. The idea is that peoples' daily lives are significantly affected by rhythmic cycles with periods of exactly a 23-day physical cycle, a 28-day emotional cycle, and a 33-day intellectual cycle.
Each of these cycles varies between high and low extremes sinusoidally, with days where the cycle crosses the zero line described as "critical days" of greater risk or uncertainty

(Source: Wikipedia)

Although it has not been scientifically proven that the biorhythm really affects an individual's life, it still can be fun to observe. This app also shows moon's phase and can store other's biorhythm.