Biorhythms are inherent cycles which regulate memory, ambition, coordination, endurance, temperament, emotions, and much more. We each have three fundamental biorhythm cycles. Each biorhythm cycle has a particular function, and a particular life cycle.
Our physical biorhythm cycle completes one life cycle in 23 days. Our emotional biorhythm cycle lasts 28 days, and our intellectual biorhythm cycle lasts 33 days. At mid point and end point in each cycle, they sharply move back to zero point and change polarity. The day a cycle changes polarity is called a transition day (also called a critical day, or caution day).
Transition days are the days when we may feel a little off, or have a downright bad day. A double transition day is when two of your cycles transition on the same day. This day may be difficult. A triple transition day is when three of your cycles transition on the same day. Triple transition days are rare, occurring once every 7-8 years.
Physical Cycle - 23 days The physical cycle is said to be the dominant cycle in men. It regulates hand-eye coordination, strength, endurance, sex drive, stamina, initiative, metabolic rate, resistance to, and recovery from illness. Surgery should be avoided on physical transition days and during negative physical cycles.
Emotional Cycle - 28 days The emotional cycle is considered the dominant cycle in women. It regulates emotions, feelings, mood, sensitivity, sensation, sexuality, fantasy, temperament, nerves, reactions, affections and creativity.
Intellectual Cycle - 33 days The intellectual cycle regulates intelligence, logic, mental reaction, alertness, sense of direction, decision-making, judgment, power of deduction, memory, and ambition.
The above explanations are borrowed from http://www.life-cycles-destiny.com/for/astrology-biorhythm-lunar-calendar-compatability.htm