Cycles of Nature: Cycles in Plants and Animals


Cycles can be seen in plants, in animals, and in nature.  Some examples of the range of cyclic variables demonstrate the variety.

Circadian cycles

Figure 2.3
Historically,many studies have been made of pregnant women to see at what time of day they went into labor.  Figure 2.3 shows the percentage of a total of over 200,000 women who began labor during each hour of the day. If labor were equally likely to begin at any hour, then 1/24 of the women (about 4 percent) would have begun
each hour. But all times aren't equally likely. The graph indicates that women in these studies were over twice as likely to begin labor about midnight as at noon. In between these times, the likelihood rose and fell along a smooth line.  It appears to show that women have underlying hormone cycles that, when gestation is complete, made labor easy to trigger during the early hours of sleep.


Figure 2.4
Figure 2.4 shows the peak times for 21 of the many cycles that have been demonstrated in humans.  The scale across the bottom giving the hour of the day, and the darkened section of the scale showing the approximate hours of sleep for the people in the studies. The black dots indicate the average peak time of the cycle for each group, and the arrows show the usual range of variation among people.

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.

Monthly 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
Figure 2.5
(relative to the length of each woman's menstrual cycle).  The dots show, on average, when in the cycle of each peak occurs. For example, whatever the length of the cycle, whether 26 or 31 days or some number in between, the peak for red cell count occurs right in the middle of the cycle.

Annual cycles

Figure 2.6
Figure 2.6 lists variables with an about-yearly cycle. [These data reflect only a few individuals.] The dots show the cycle peaks of each variable along a time axis from January through December. The hormone prolactin, for example, peaks in early spring, while the hormone testosterone reaches its peak in early fall. All three of these maps illustrate the point that biological variables are cyclical, and that different variables can peak at different times.

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.
Figure 2.7
For example, the circadian temperature cycle in human beings and the temperature cycle which accompanies the female menstrual cycle, together, make up the observed body temperature cycle. Figure 2.7 is a record of a woman's body temperature taken three times a day for four months. You can see both the circadian variation and the monthly movement up and down of the midline, modifying the daily cycle. Figure 2.8 demonstrates how the interplay among two cycles results in a combined cycle that is the algebraic sum of the variations in the individual cycles.
Figure 2.8


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.