Biorhythms are natural, rhythmic cycles controlled by our "biological clock." Present from birth until death, it is believed biorhythms--or biological rhythms--occur independently of our will, health, stress, or other external influences and that they do not vary from person-to-person.

Four biorhythmic cycles have thus far been identified: physical biorhythms cycle every 23 days, influencing our coordination, energy, strength, endurance, initiative, and resistance to illness. Emotional biorhythms cycle in 28 days, affecting relationships, moods, sensuality, and feelings. Intellectual biorhythms cycle in 33 days, influencing cognitive, critical thinking, and problem-solving abilities; reasoning; learning; memory; and alertness. Intuition biorhythms cycle over 38 days, making us more or less receptive to "sixth sense" perceptions such as instinct and hunches. Each biorhythm cycle has exactly the same capacity--from 100% to minus 100%. During the positive swing, energy levels and effectiveness in that particular cycle are high and we may find ourselves having really "good days" or handling problem situations smoothly. During the negative cycle, the converse is true, and we often have "bad days" or find problems difficult to handle. As a cycle rises from negative to positive, it crosses a mid-point. This day is called a "neutral day." As it crosses the midpoint moving from positive to negative, this is a "day of crisis." On both of these days, capabilities could be either extremely high or extremely low. Biorhythmic cycles are calculated mathematically based on date, month, and year of birth. Those who study biorhythms suggest important tasks relative to each particular cycle be planned for the positive days and avoided during the negative days, and that extra care be taken during the negative swing to avoid accidents. Therefore, knowing one's biorhythm cycle may be helpful in planning critical tasks and understanding otherwise unexplained highs and lows.

While the existence of biorhythms has not been proven scientifically, empirical evidence strongly suggests their influence is universal. Astute observers--probably from the beginning of time--were aware of cyclic rhythms in humans. In a book entitled Is This Your Day? published in 1973, George S. Thommen delves into the history of biorhythm research, referring to studies by Dr. Hermann Swoboda, professor of psychology at the University of Vienna, volumes published in the early 1900s by Dr. Wilhelm Fliess of Berlin, and other writings. Even Hippocrates apparently noted cyclic rhythms in his patients, taking them into account when planning treatment.