Many people report that they can improve the quality of their lives by monitoring the highs and lows of these cycles and acting accordingly. For example, you might try to schedule important exams during your intellectual highs, avoid talking to your significant other during your emotional lows.

When the curve of any cycle is above the midline, we experience a "high" in the corresponding field (physical, emotional or mental). If it is the physical curve, you will feel bodily well and, for example, if you were to have an operation the body would be more resilient. If it is the emotional curve, you will be in a good mood and your contact with other people will be positive. If it is the mental curve, your thought processes will be sharper, concentration will be easier and you may have a lot of ideas and insights.

When the curve of any cycle is below the midline, we experience a "low" in the corresponding field. If it is a physical low, you may feel weak and listless; if it is an emotional low, you may feel miserable and out of sorts; if it is a mental low, thinking can be foggy and concentration hard.

When any of the curves crosses the midline, this is a critical time in the corresponding field and if two curves cross the midline at the same point, this is a very critical time. Critical in the physical field means you may feel exhausted and it is better to avoid physical effort and operations. Critical in the emotional field means you may feel irritable and sad, and emotional outbreaks are likely so it is better to avoid conflicts and arguments. Critical in the mental field finally means sluggish thinking and it is better to postpone activities that demand a lot of concentration.

Biorhythms can't predict or explain events, they can only suggest how we may deal with them. For example, some doctors plan operations around them or sports coaches use them in their individual training programs. But it is up to you whether you believe in them or not.

Biorhythms SkepDic.com (link)

The theory originated in the nineteenth century with Wilhelm Fliess, a Berlin physician, numerologist and good friend and patient of Sigmund Freud.  Fleiss was fascinated by the fact that no matter what number he picked he could figure out a way to express it in a formula with relation to either 23, 28 or both.   The latter number he associated with menstruation and thus when he was convinced that all the world is governed by 23 and 28, he called the 28-day period "female" and the 23-day period "male." In 1904, several years after Fliess's discovery, Dr. Hermann Swoboda of the University of Vienna, claimed he discovered these same periods on his own. In the 1920s, Alfred Teltscher, an Austrian engineering teacher, added the 'mind' period of 33 days, based upon his observation that his students' work followed a 33-day pattern. The theory was popularized in the 1970s by George Thommen (Is This Your Day? How Biorhythm Helps You Determine Your Life Cycles) and Bernard Gittleson (Biorhythm--A Personal Science). Neither book provides scientific evidence for biorhythms. They consist of little more than speculation and anecdotes. However, by now the static idea of periods was replaced with the dynamic notion of cycles, which are now known as the physical, emotional and intellectual cycles. Interestingly, not only did the "female" period become the emotional cycle, but both men and women are said to share the same physical and emotional cycles of 23 and 28 days respectively. One might have expected that, given the different hormonal natures of males and females, the sexes might have at least some unique and distinct rhythmic cycles.