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 were 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 tends 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 resynchronization 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 resynchronization 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
Light Therapy for Seasonal Affective Disorders
One of the most important applications of light therapy is in the treatment of Seasonal Affective Disorders (SADS). There is a large body of scientific evidence that points to the efficacy of light therapy for the treatment of SAD. What is not quite understood, yet, is how light treatment works.
We have shown that our daily rhythms are affected by the availability of the natural sun light. Many of us work in artificially lit buildings and does not get enough light. Most artificial lighting cannot replace the natural light. The reason for this is that the type of indoor lighting we use is not of sufficient intensity to affect the hormonal mechanisms which control our bodily rhythms. Intensity of light is measured in a unit called lux. One lumen means the light received by the receptor at an intensity of one lumen per square meter. Thus the intensity of light at any point is determined not only by the strength of the illumination source but also by how far it is from the source. The electric light used in most homes and workplaces rarely exceeds 500 lux. A sunny afternoon could be as much as 100,000 lux, and even the cloudiest day is rarely below 10,000 lux. Researchers have discovered that light of at least 2,500 lux is necessary to suppress melatonin production in humans. Most of the bright light therapy uses 5000 lux light (10,000 lux preferred.) The artificial light we use indoors is not of sufficient intensity to suppress melatonin and to correct the circadian rhythm. Night-shift workers, and people who live in Arctic climates, are usually exposed to light levels of only 50 lux. Light specialists believe this "mal-illumination" may be at the heart of many common disorders, including fatigue, depression, skin damage, suppressed immune function, and, of course, sleep problems.
Light therapy for Seasonal Affective Disorder and circadian-rhythm disorders involves sending visible light through the eyes so that it reaches, and triggers, the pineal gland.
There are several different forms of light therapy in use today; the oldest is sunlight itself. The sun is the ultimate source of full-spectrum light, which means it contains all possible wavelengths of light, from infrared to ultraviolet. Generally speaking light therapy involves the use of equipment that sheds either full spectrum or bright white light.
In most cases, the purpose of light therapy is to increase the amount of light to which we would otherwise be exposed. Bright light therapy consists of looking at special broad spectrum bright lights from one-half to three hours a day, generally in the early morning hours. One should not stare directly into the lights because of possible eye damage.
A substantial amount of light is needed, which means the distance from the lights to your eyes needs to be monitored---close enough to give you the best amount of light, but distant enough so you don"t hurt your eyes.
By the mid- 1980s, bright-light therapy (phototherapy) had become the treatment of choice for SAD. But many people found it difficult to allocate the four hours everyday for the light therapy. Additional studies were conducted by the researchers to determine an optimum light therapy. It was found that similar benefits can be obtained from a morning only therapy, effectively slashing the time by half. Later, by increasing the brightness or the intensity of the lights used, the therapy time was cut down further.
A few years later, it was cut further by brightening the lights. For example, with 10,000-lux light, it only required 30 minutes of exposure per day to get effective cure from SAD.
SAD symptoms typically begin to lift about a week after the start of phototherapy. But they return shortly after discontinuing the treatment. As a result, authorities urge people with SAD to sit under bright light daily from October through April
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