"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, chudler@u.washington.edu. 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, dan@sfn.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.