Cycles can be seen in plants, in animals, and in nature. Some examples of the range of cyclic variables demonstrate the variety.
The cycle peaks for most of the variables in the "general" list occur between noon and 6 p.m. — in the middle of the waking hours. These variables correspond to task performance and coordination. Temperature and blood pressure are related to the rate at which your body functions can work, while the other four are indicators of actual performance.
The exception among the "general" variables is the rate of cell division in skin, which peaks soon after the beginning of sleep. Cell division does not directly contribute to performance, but is part of growth and repair.
Variables in the lower list are for some of the contents of blood and have peak times for cycles across all 24 hours of the day. You can see that the cycles in red and white blood cell counts are opposite — the peak times are 12 hours apart. All of the cycles in Figure 2.4 are called "circadian" Latin for "about daily" cycles.
Similar maps can be made for longer cycles. Both men and women have detectable cycles with periods of about three to six weeks, but the most obvious cycle in this range is the menstrual cycle in women. Figure 2.5 shows common peak times for 11 about-monthly cycles
Since there are also variations in cycle peaks among individuals, it is not always wise to make estimates based on information taken from measurements of groups of people. However, there are some kinds of cycles, such as the labor-onset cycle, that can be seen only in a collection of people.
Complex Cycle Interactions
Another fascinating aspect to cycles, lending insight into the complex mystery that is life, is the way one biological variable can be influenced and shaped by cycles of different lengths.
Biological processes are influenced by many cycles, from those only fractions of a second in period to those with periods of about a week, several weeks, a year, and even longer. The next section explores the interactions between cycles and the forces that influence their functioning.
Not Everything has a Cyclic Aspect
There are, of course, steady trends in biological variables as well as cycles. In humans, height, elasticity of tissue, and stiffness of the lens of the eye are variables which generally progress in one direction. How many others can you think of?
What factors cause these rhythmic patterns in variables? Can they be changed, or adjusted? These are questions scientists are trying to understand. There are both biological (internal to the organism) and environmental factors.