It looks like every civilization invents own biorhythm cycles that slightly differs in periods ...The Moon played an important role in ancient Mesoamerican (middle America, i.e. Mexico and northern Central America) astrology. The astrologers of this region noticed that blocks of time had different and specific qualities. They recognized that the rising and setting of the Sun every day establishes the basic time unit — the day. Their great discovery was a 20-day cycle, similar in many ways to a biorhythm. Each day in the 20-day cycle has specific qualities, similar to the signs of the Western zodiac. Each has a name, a symbol, and a delineation.
They also noticed how fast the Sun and Moon moved across the sky. In one day, the Moon moves as far as the Sun does in 13 days. Also, in one year, the Moon cycles with the Sun 13 times. These are two major reasons why the number 13 came to be the most powerful number in ancient Mexican astrology.
The most powerful calendar of ancient Mexico was one that had 20 “weeks” of 13 days, a total of 260 days. This astrological calendar, or almanac, was (and still is) used for interpretation and forecasting, not for ordinary use. The twenty 13-day “weeks,” known in Spanish as the Trecena (13ths), are basically lunar in nature since they represent the time taken for the Sun to match the Moon’s daily travel. Each of the twenty 13-day periods has a specific meaning that is “generated” by the named solar day (the day-sign) that begins the period. These “first days” are numbered 1, followed by the name of the day.
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