All forms of life respond to the cycles of the sun, the moon and the changing seasons. This is called the biological clock or circadian rhythm. The word circadian comes from the Latin words circa, meaning about, and dia, meaning day. Research has revealed that the body clock operates on a twenty-five hour schedule, but plants and animals have the ability to adjust their varying cycles to the twenty-four hour solar day.

In the brain is a type of pacemaker called the suprachiasmatic (soo-prah-ki-az-MAT-ik) nuclei. (reference source: This area of your brain regulates the firing of nerve cells that seem to set your circadian rhythms.

Scientists can't fully explain how this area of the brain keeps time. They do know the brain relies on outside influences, zeitgebers (ZITE-ga-berz), to keep it on a 24-hour schedule. (reference source to zeitgebers: Psychiatric Times. May 1996. Vol. XIII. Issue 5)

The most obvious zeitgeber is daylight. When daylight hits a person's eyes, cells in the retinas signal the brain. Other zeitgebers are sleep, social contact and even regular meal times. They all send timekeeping clues to the brain, helping keep circadian rhythms running according to schedule.

Circadian rhythms keep the body alert during daylight hours and help it to relax when night falls. This inner clock will even awaken a person if they forget to set their alarm. Unfortunately, it will also awaken them on days when they could have slept in.

Circadian rhythms affect hormones. Cortisol affects various body functions, including metabolism and regulation of the immune system. Its levels are highest between 6 and 8 a.m. and thereafter decline gradually throughout the day. If a person's daily sleeping schedule changes, the peak of cortisol's cycle changes accordingly.

Growth hormones stimulate growth in children and help maintain muscle and connective tissue in adults. Growth hormone production is triggered during sleep, and peaks during the first two hours of sleep. Sleep deprivation inhibits the production of growth hormones. (The Lancet 1999;354:1435-1439.)

Circadian rhythms also influence body temperature to a degree. Body temperature is lowest when a person is inactive and higher during physical activity, but circadian rhythm can raise or lower temperature independent of physical activity. During the late afternoon, body tempreature can be as much as 2 degrees Fahrenheit higher than in the morning, regardless of outside stimulus. Similarly, blood pressure rises in the morning and remains elevated until late afternoon, independent of physical activity. Studies show pain tolerance is greatest in the late afternoon.

Many things can upset the rhythm of the body. One of these is called jet lag. This affects people who travel across several time zones, often arriving at their destination at an hour earlier than when they left home. Jet lag upsets the sleep/wake cycles, causes headaches, irritability and a general feeling of malaise.

The importance of the circadian timing system cannot be stressed enough. It is involved in the 24-hour variation of virtually every physiological and psychological variable researchers have studied.

This type of rhythm exists in all living things. Humans have circadian rhythms. So do animals, insects, and even plants. They reflect the adaptation of organisms to the many changes that are associated with the rotation of the earth on its axis. These things include light, air pressure, and temperature. There are over 100 bodily functions and behaviors that follow circadian rhythms. These include such things as blood pressure, heart rate, appetite, secretion of hormones and digestive enzymes, sensory acuity, elimination, and the body's response to medication.

In general, our bodies adapt a strict 24-hour schedule. The reason is the external time cues that are all around us. They train our biological rhythms. In order for scientists to identify endogenous circadian rhythms a subject must be isolated from sunlight, clocks, environmental sounds, and to all other cues of time. It has recently been shown that in primate infants, even at a very premature stage, low intensity lighting can regulate the developing clock.

The body’s overall coordinator is the biological clock. This controls the circadian rhythms. It is located in a teardrop shaped cluster of cells in the hypothalamus. This is the SCN (suprachiasmatic nucleus). There are two, one behind each eye. In the back of the eye there are special receptors that transmit information through neural pathways that allow the body to respond to light and dark changes. This causes the SCN to send out a message to the brain to make the body adapt to the changes. This is not the only clock in the body. They are scattered throughout the body. Some operate independently of the SCN, but for most of the circadian rhythms, the SCN is the master pacemaker. It is now thought by scientists that the circadian timing system develops parentally. The SCN is now thought to be present in primates by mid-gestation.

The SCN regulates the fluctuating levels of certain hormones and neurotransmitters. These neurotransmitters and hormones provide feedback that affects how the SCN functions. The ebb and flow of circadian rhythms is not controlled only biologically, there are external cues as well. The most important of these is sunlight.

Circadian rhythms, under normal circumstances can be synchronized just like a wristwatch can. When a person’s (or other living being’s) routine is changed, their circadian rhythms may be thrown out of phase with one another. This is internal desychronization. This often happens when a person is “jet lagged.” Sleep patterns can adjust fairly quickly but temperature and hormone cycles may take several days to get back to normal.

Sources: Developing Circadian Rhythmicity in Infants by Scott A. Rivkees, MD
Psychology Seventh Edition by Carole Wade and Carol Tavris
The World of Psychology Fourth Edition by Samuel E. Wood and Ellen R. Green Wood

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