Biorhythms
There is little, if any, scientific support for this theory, which claims there are three precise cycles that control human behavior. These are: a physical cycle of 23 days; an emotional cycle of 28 days; and an intellectual cycle of 33 days; Hazardous critical days are proposed to occur when two or more cycles intersect.
In contrast, biological rhythms such as activity cycles, feeding cycles, and sleeping cycles, are well-known. They vary from individual to individual and most are tied to the 24 hour rotation period of the Earth. Biological rhythms are real; biorhythms are a hoax.