Rhythms and Body Clock
From the Latin circa (about) dies (a day), the circadian rhythm is the twenty-four-hour cycle of light/dark, wakefulness/sleep to which most human physiologic processes are set. At regular intervals each day, the body tends to become hungry, tired, active, listless, energized. Body temperature, heart-beat, blood pressure, hormone levels, and urine flow rise and fall in this relatively predictable, rhythmic pattern - a pattern initiated and governed by exposure to sunlight and darkness.
Experiments where humans were placed in isolation chambers, cut off from all potential environmental cues, have shown that, in the absence of natural daylight, rhythms are still maintained. But in the absence of the day light, the rhythms tend to deviate from 24 hours. For instance, the rhythms was found to expand to 24-30 hours, thus disrupting the biological processes over a long period of time.
The fact that animals and humans can continue to function according to daily and annual rhythms in the absence of external environmental stimuli means that animals and humans possess some kind of biological clock, which act as a backup mechanism in case it cannot get the proper stimuli from the natural events such as sunshine.
This behavior can be illustrated by our clocks. Let us say, our clock is running slow. Over a period of time, the clock may lag the actual time because of this defect. Usually, we will reset the clock when it gets far out of sync by other external stimuli like a radio or phone time. Now, if we do not have access to this external synchronizing signal, the clock can get far out of line with the reality. Our body clocks functions the same way. The biological clock can keep the time; but in the absence of correction from the day/light cycle provided by the sun, the biological clock tend go out of sync affecting our physical and mental health. A similar thing happens when we travel across time zones; we tend to experience what is known as "jet lag".
However, in the absence of natural light our body clocks may lose or gain a little time. This in turn could lead to the desynchronization of different rhythms. For example, in the absence of sufficient environmental light the sleep-wake and associated rest-activity rhythms may lengthen to a cycle of between 30 and 48 hours, while the temperature rhythm may remain at a period of, say, 25 hours. Such desynchronization of the body's intricate rhythms is suspected to trigger problems: hormonal imbalances, sleep disorders and mood disturbances.
Circannual rhythm is the annual or yearly cycle used by all living things.
Circaseptan rhythm is a seven-day cycle in which the biological processes of life, including disease symptoms and development, resolve. Many physicians believe that transplant patients tend to have more rejection episodes seven, fourteen, twenty-one, and twenty-eight days after surgery. They further believe that medications administered to the patients at particular times may be more effective than at other times. These are all related to the circaseptan rhythm.
How does the brain know when it is light or dark?
Deep within the brain, inside the hypothalamus, lie two clusters of cells (i.e., neurons) called the suprachiasmatic nuclei (SCN). Each of these SCN is composed of more than 8,000 neurons. The SCN act as the body's circadian pacemaker. In mammals, the SCN appear to get their information from photoreceptors in the retina, which transmit signals about light and dark through the optic nerves to the hypothalamus. Once these messages enter the SCN, a series of physiological reactions takes place.
What happens after the light/dark signal reaches the SCN?
We are not sure. The pathway from the retina through the optic nerves to the SCN extends further to reach the pineal gland, which lies adjacent to the hypothalamus above the brain stem. Stimulated by the message it receives from the SCN, the pineal gland either secretes its main hormone, melatonin, or inhibits melatonin's release, which may result in the production of serotonin as was explained before.
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