II. Sleeping and Dreaming
A. Biological clocks and circadian rhythms
We all go through the sleep-awake cycle everyday, on most days. But the methods we use to determine when to sleep and when to get up vary. While some people go to and get out of bed at set times, those who are less disciplined often let their work or play get in the way of keeping their regular schedule. While most people begin their sleep at some time during the night and awake during one of the daytime hours, a few do a consistent reversal. While we use electric lighting to push our bedtime back and the sound of an alarm electronic alarm sounds to wake us up, our ancestors relied on natural light. But have you ever thought about this question: What would happen if there were no outside time cues, neither electronic nor natural?
To find out, researchers have put their subjects in "timeless" environments without sunlight, clocks, or radios. The consistent finding in these studies is that people, even when living in a timeless environment, still maintain a sleep-wake cycle. This indicates that we have an inner biological clock that regulates the sleep-wake cycle.
Scientists have revealed that there is more than one biological clock in the human body regulating different types of biological processes. Some are set for a longer cycle than others. For example, our skin completely alters itself every six weeks and women's menstrual cycle is about 28 days. There is a group of the biological clocks that are set for periods between 24 and 25 hours. These are called "Circadian Rhythms" (circa means about in Latin and dia means day). One particular circadian rhythm controls our sleep-wake cycle. Others control our body temperature, all the vital signs, some hormonal releases, and even bone length.
The human natural circadian rhythms vary from person to person. But on average, they are set to a period that is longer than 24 hours, but probably shorter than 25 (Researchers debate about the exact length). That leads to some questions:
If our internal clock (i.e., the human circadian rhythms) runs on longer-than-24-hour days, why are we living our lives on 24-hour days? And does this discrepancy cause any problems for us, health wise mostly?
To answer the first question, you have to understand how the 24-hour days we live came about. It is based on an external clock -- every 24 hours, the Earth completes one rotation. There are many external clues of this 24-hour time set, the most obvious of which is the light-dark cycle. Humans then invented mechanic, electric, and digital clocks running on 24-hour cycles to mimic the external clock.Scientists are not sure why there is a discrepancy between the internal and the external clocks. Some speculate the discrepancy gives us room to adjust our internal clock based on outside clues, such seasonal changes. The fact that humans generally have no problem with the discrepancy indicates that although our circadian rhythm is biologically set, it also has built-in flexibility to be reset by natural elements, such as natural light. We do create trouble however when artificial means are employed to interfere with or force changes on our circadian rhythms. Studies have shown increases in cases of peptic ulcer and cardiovascular mortality among shift workers. Nicotine addiction, chronic fatigue, excessive sleepiness, and difficulty sleeping, depression, and other psychological and social problems are also found to be more common among people who work shifts.
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About Sleep and Dreams:
A. Biological clocks and circadian rhythms
B. Sleeping
C. Why Sleep Is Important
D. Dreaming and Dreams
http://psychology.about.com/library/bl/bl101_3.2a.htm
