There are three cycles that begin the day we are born (known as our birthday; learn something new everyday), and those three cycles make up our biorhythm. The biorhythm consists of the physical, emotional, and intellectual cycles, and those cycles have highs, lows, and neutral points.
The high point of an individual cycle is the day that you are at an optimum level for that cycle. For example, when your physical cycle is at its peak, you can physically perform better than on other days. The same goes for the intellectual and emotional cycles; peaks are when you feel the best for those qualities. On low days, while it is the opposite of high days, and you don't perform well in that area, they are not bad. Your body is just resting in that area. It is the critical days where something bad could happen. For example, if you were in a race on a critical day, there is a higher chance you could fall and get hurt. But don't forget, there are two other cycles. While you might be having a critical day or be low for one, you could be high for one or both of the others.
Sometimes, rarely, there are days where two or even all three of your cycles are critical! These are called double and triple critical days, and the possibility of something bad happening is, well, twice or three times more likely than one a regular critical day. If you know your biorhythm, you can plan around your cycles, and literally optimize your life.
How do you measure your biorhythm? As was stated, all three of your biorhythmic cycles begin the day you are born. The physical cycle lasts 23 days, the emotional cycle lasts 28 days, and the intellectual cycle lasts 33 days. The cycles are represented by the sine function (that's trigonometry). To effectively plot a sine function and find your biorhythm graphs, the first step is to find the number of days you are alive. The #1 mistake in finding the number of days you were alive is not counting leap years. If you don't count them it's no big deal, but if you don't count them, of course your number is not exact. After finding the number of days you were alive, divide that number by each cycle: 23, 28, and 33 (physical, emotional, and intellectual). You'll have to do it by hand or get a calculator that works with remainders because (well its obvious now) the number needs to have a remainder. Unless it's an exact division, then it won't have a remainder. After you do that, you'll have the number of cycles that you've completed throughout your life.
Since the rest can't be explained without going into deep detail about trigonometry, this is where the article ends. However, if you really want to see what your biorhythm curves look like, there are many tools online that will show you.