According to the theory, our lives are affected by three primary biorhythms: emotional, physical and intellectual. Others claim that there are also secondary biorhythms (spirituality, intuition, consciousness and aesthetics) some of which may be combinations of the three primary. Biorhythms begin at birth and are oscillating between positive and negative phases throughout life.
Supporters of biorhythms state that when a biorhythm is in a positive phase, people may feel or be able to perform better, biorhythm related activities. Conversely, when a biorhythm is in a negative phase, you may experience a bad mood or difficulty performing activities related to this biorhythm. When a biorhythm cross the middle line is considered to be in a "critical" moment.
Emotional (28 days)
Affects: the humor, sensitivity, creativity
Physical (23 days)
It Affects: strength, endurance, coordination, wellbeing
Intelectual (33 days)
Affects: analytical thinking, logic, learning ability, memory
Spiritual (53 days), Intuition (38 days), Awareness (48 days), Aesthetics (43 days)
More Info (Wikipedia)