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"Circadian rhythm" is an expression often used in the sleep research field. The word circadian comes from the Latin circa diem, meaning "about a day." The circadian rhythm is a 24 hour biological cycle. All animals and most plants posses this and many other biological clocks.
At the base of a human brain is the hypothalamus , a pinhead-sized structure. The suprachiasmatic nucleus of the hypothalamus serves as a biological clock that regulates the sleep-wake cycle . Information about light travels from the retina to the hypothalamus through the nerves. The hypothalamus continually measures the light exposure, and accordingly adjusts the timing of the sleep-wake cycle. For this reason, daylight savings time (5) and solar eclipses (6) present challenges to our biological clocks.
To prove that our circadian rhythm isn't just a reaction to external stimuli like daylight, alarm clocks or watches, but much more an internal biological action, two German scientists conducted an experiment in which they locked up volunteers in a bunker where no sunlight could enter. (1) The volunteers were kept there for one month without having any idea whatsoever of the time it really was outside and were encouraged to go to bed whenever they wanted. The results of the experiment after one month showed that the natural repetitive cycle for these volunteers came to "settle" at above 24 hours, at about 25 hours (of which one third was spent asleep)!
In a human experiment, people were left in a cave and provided with electricity which they were allowed to control. The humans gradually adjusted to a 48 hour cycle, with 18 hours of sleep and 30 hours of activity. (2)
Recent research has discovered that humans may have a circadian rhythm closer to 24 hours than previously thought. See our news reports.
For more information about circadian rhythms, we recommend the University of Virginia's Center for Biological Timing website. It includes Educational Outreach Activities , a Biological Timing Tutorial , and a fascinating Biological Clock Cell Animation ! (3) Dr. Gene Block travels throughout the state to encourage students to become involved in these science experiments, but other schools can sign up through the Web.
See also five activities and experiments on Circadian Rhythms and Body clock targeted for grades 3 -12 developed by Dr. Eric Chudler, University of Washington, in his neuroscience website. There are other pages of activities on sleep, dreaming, and other neuroscience topics.
(1) Hobson, J. Allan. [Sleep. German] "Schlaf: Gehirnaktivität im Ruhestand". Aus dem Amerikan. übers. von Ingrid Horn. [Translated by Ingrid Horn]. Heidelberg, Spektrum der Wissenschaft Verlagsgesellschaft mbH, 1990, pp. 44.
(2) Nightwatch: the Natural World from dusk to dawn. New York, Facts on File. 1983.
(3) Permission to Link to CBT website at: http://www.cbt.virginia.edu/index.html granted. Permission for use of photograph granted, by Dr. Gene Block while visiting Thomas Jefferson High School for Science and Technology, VA, where this photo was taken during a presentation on Circadian Rhythms, 1999.
(4) Permission to Link to Dr. Chudler's Web Pages at this site: http://faculty.washington.edu/chudler/neurok.html . Email message. Eric Chudler, email@example.com. August 9, 1999.
Dr. Chudler's page also led US in August to the Society For Neuroscience's excellent website, which you may wish to explore for more information on Circadian Rhythms. http://www.sfn.org/briefings/bio_clocks.html Permission for link & brain / bioclock image found at the above URL; Daniel Kapusta, firstname.lastname@example.org. August 11, 1999. Email message.
(5) "Don't Lose one more hour of sleep! Daylight savings time sleep tips offered by National Sleep Foundation." Washington, National Sleep Foundation press release, Apr. 2, 1999, 2 p. This recommends to prepare for the time change (4 Apr. 1999) by "Go to bed one-half hour earlier" and "sleep at least one-half hour later" to adjust, as "Americans already get too little sleep."
(6) Majendie, Paul M. "Eclipse effect on nocturnal animals." Reuters wire services, Aug. 9, 1999. from NewsEdge. "Birds stop singing, badgers leave their lairs and bats fly into new-found night when a total eclipse of the sun turns the natural order upside down."
(7) http://www.sfn.org/briefings/bio_clocks.html "Light causes the brain's internal clock to reset its cycle. Sunlight reaching photo receptors in the retina travels to the brain by the optic nerve. It sets off reactions in a region of the hypothalamus called the suprachiasmatic nucleus (SCN), which serves as the body's clock. Circadian changes in the SCN affect the nervous system and cause daily fluctuations in many body traits. Nerve fibers also carry signals from the SCN to the pineal gland, which affects hormones and other functions." Illustration by Lydia Kibiuk, Copyright © 1995 Lydia Kibiuk.[an error occurred while processing this directive]