It should slowly be becoming obvious to you now that the very essence of life and nature is rhythm. Rhythms can be found in the regular ebb and flow of the ocean tides, as well as in the alternating cycles of darkness and light that occur every 24 hours.

The French astronomer Jean Jacques d'Ortous de Mairan (A mouthful, I know haha) first observed a rhythm in his house plants in 1729.

He noticed that the plants closed their leaves in the evening and opened them again each morning. As an experiment, he placed one plant in total darkness for a period of time.

He was amazed to discover that the plant continued to close and open its leaves regularly even though it was not exposed to the sunlight.

Henri-Louis Duhamel, an engineer, and agriculturalist possessed a scientific mind but did not believe the experiments of de Mairan. How is it possible, Duhamel questioned, for a plant to maintain a regular sleep pattern if the plant does not know whether it is night or day?

He decided to duplicate the total darkness experiments of de Mairan. And to make sure that the plant received absolutely no light, he placed the plant in a leather trunk, which he covered with blankets and his in a closet.

Still, the plant's leave closed in the evening and opened in the morning.

Duhamel, more puzzled than ever, now wondered what other stimuli could be responsible for the plant's sleep pattern. The night is much cooler than the day, he reasoned.

Could the stimulus be a change in the temperature?

Duhamel placed his plant in a hothouse and raised the interior temperature well above normal. He recorded the results of this experiment in 1758:

"I have seen this plant close up every evening in the hothouse even though the heat of the stoves had been much increased. One can conclude from these experiments that the movements of the sensitive plant are dependent neither on the light nor on the heat"

Scientists studied this strange rhythm over the years. In 1939 Erwin Bünning, a German botanist published a hypothesis that at first generated little interest.

Bünning had determined through studies and experiments with leaf movement that plants have internal circadian (24-hour) rhythms, and these rhythms are used to measure time.

In effect, Bünning theorized that plants possess biological clocks. Other scientists of the time thought the Bünning hypothesis too absurd to be believed. Not until many years later through the accumulated work of other researchers, did evidence point to the possibility of biological clocks in organisms.

From the lowly one-celled plant all the way to human beings like you and me.

The internal world of a human being is influenced by the beat of nature. Chronobiologists (biologists who study how the body measures time) have identified four body rhythms in human beings: ultradian rhythm, circadian rhythms, circamensual rhythm, and circannual rhythms.

Ultradian rhythm is a 90- to 100-minute cycle. Studies have shown that daydreaming, levels of concentration, and feelings of hunger rise and fall in ultradian rhythm throughout the day. The sleep pattern is also ultradian.

There are stages of sleep through which one passes during a normal night. As one's eyes close on the world, alpha waves are produced in the brain. Alpha waves represent a state of relaxation. 

The myoclonic jerk, a sudden body spasm, may jolt the sleeper awake for a moment. This normal occurrence is the result of a spurt of brain activity. 

Slowly one drifts into Stage One, a time of slight sleep. Muscles begin to relax, pulse and breathing begin to slow, and body temperature falls.

In Stage Two, one's eyes slowly roll from side to side and thoughts are fragmented; one has been asleep about 10 minutes.

As one descends to Stage Three, muscles are relaxed and body temperature and blood pressure continue to fall. The heart rate slows and breathing is even.

One gradually sinks into Stage Four, which is known as delta sleep. This is the deepest level of sleep. The term delta sleep is derived from the fact that, at this point, the brain produced large, slow brain waves - delta waves.

Having reached the bottom of the sleep cycle, one begins to ascend to the stage of light sleep. Instead of one's waking up, though, the eyes begin to dart back and forth.

This period of rapid eye movement (REM) is characterized by dreaming. After the REM period ends, one again descends through Stage Two, all the way to delta sleep, and back up again for another interval of REM.

This cycle recurs about every 90 minutes during the night.

The name circadian rhythm comes from the Latin Circa, meaning "about," and dies, meaning "a day." Body temperature fluctuates one or two degrees Fahrenheit (less than one-degree Centigrade) during the 24-hour circadian period.

This fluctuation determines if ones is a "lark" or an "owl." Larks are people whose body temperature is high in the morning. Larks leap out of bed at the sound of the alarm clock, ready to face the day.

Owls, on the other hand, would prefer to roll over for another forty winks. Their body temperature rises more slowly and peaks later in the day.

Other bodily functions, such as breathing, blood pressure, blood sugar level, and pulse rate, also vary in circadian rhythm.

People who travel by plane across many time zones often experience a phenomenon called desynchronization (jet lag). Travelers may arrive at their destination just in time for breakfast, but their biological clocks may be telling them that it is time for bed.

They may also be fatigued and not very alert for several days while their circadian rhythm adjusts itself to the new time period.

Circamensual refers to the 28- to 30-day rhythm associated with female menstruation. There also seems to be a monthly rhythm associated with males.

In the seventeenth century, a doctor named Sanctorius began weighing men as an experiment. He found that a monthly one- to two-pound change was evident in the weights of his subjects over a period of time.

Modern research has hinted at the possibility of 4- to 5-week cycles of hormonal and mood changes in men also corresponding to the menstrual cycle.

The term menses means "lunar month." Experiments at the Rock Reproduction Clinic in Boston have strengthened the theory at menstruation is controlled by the phases of the moon. 

In the study, women with irregular menstrual periods were asked to sleep in the indirect light of a 100-watt lamp (simulating a full moon) from the fourteenth to the seventeenth nights of the cycle following the onset of menstruation. 

A gradual regulation of the menstrual period began with ovulation occurring at the time of exposure to the light. Could this regulation of the menstrual rhythm be the birth control method of the future?

The Fourth rhythm is circannual, or yearly. This rhythm in human beings follows the tempo of the seasonal changes in nature. Children grow rapidly in the spring. The sweltering heat of the summer is eased by the secretion of a thyroid hormone. 

There is evidence that late autumn and early winter are the seasons when a man's beard grows rapidly. Robert Sothern, a scientist associated with the Chronobiology Laboratory of the University of Minnesota, thinks that this is related to fur-bearing animals growing thicker coats as they prepare for winter.

Dr. Alain Reinberg of the Rothschild Foundation Hospital in Paris has done studies of human sexual rhythm. He discovered that male hormone levels and sexual activity peak during the late autumn and early winter.

These results parallel studies of the start of menstruation in girls, which usually occurs at the same time of year. These results may suggest a natural period for human conception.

There also appears to be a circannual rhythm for death.

Statistics gathered bu the professors Michael Smolensky and Franz Halberg of the University of Minnesota and Frederick Sargent of the University of Texas at Houston indicate that deaths from respiratory and heart disease are at their highest point from December through February in both the northern AND southern hemispheres.

Scientists have also determined that deaths from pneumonia and flu reach their height during a certain period across the United States.

This includes those states such as Florida that have mild weather year-round and northern states such as Minnesota that have extreme winter and summer conditions.

In explaining the scientist' conclusions, Dr. Alain Reinberg writes;

"If the risk of mortality for certain lung diseases is higher between late December and late February - In the northern hemisphere, it is not necessarily because cold and stormy weather occurs at this time, but rather because the human organism is the more susceptible to this type of infection than at any other time"

Where inside us is the timepiece that regulates these body rhythms?

Scientists don't know for sure still. Some chronobiologists are of the opinion that each cell of an organism contains its own internal clock.

Other scientists refute the internal clock theory and hold that body rhythm are controlled by external forces such as light and temperature.

Still, others believe two clocks exist, internal and external.

The theory is that every organism keeps its internal clock from running too fast or too slow by synchronizing with an eternal force. Exposure to light, such as the 24-hour sun clock, will allow the internal clock to reset itself to be on time with the rest of nature.

This may also be true for the three life cycles - Physical, emotional and intellectual - of the biorhythm.