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: http://rprcsgi.rprc.washington.edu/neuronames/glossary/suprachiasmatic_nucleus.html) 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.