Derived from the Latin words "circa" (about) and "dies" (a day), the circadian rhythm is the twenty-four-hour cycle that governs most of the body's physiological processes. Throughout the day, the body tends to experience various states such as hunger, fatigue, alertness, lethargy, and energy. Physiological factors like body temperature, heart rate, blood pressure, hormone levels, and urine flow follow a relatively predictable, rhythmic pattern. These patterns are initiated and regulated by the exposure to sunlight and darkness.
Experiments involving human isolation chambers, where subjects are disconnected from external environmental cues, have revealed that even in the absence of natural daylight, biological rhythms persist. However, in the absence of natural daylight, these rhythms tend to deviate from the 24-hour cycle. For instance, rhythms have been observed to expand to 24-30 hours, which can disrupt biological processes over an extended period.
The existence of these daily and annual rhythms in animals and humans in the absence of external environmental cues suggests the presence of a biological clock. This biological clock acts as a backup mechanism when the body cannot receive appropriate stimuli from natural events like sunlight.
This concept can be illustrated using our clocks. Suppose a clock is running slowly; over time, the clock may drift from the actual time due to this issue. Normally, we reset the clock when it falls out of sync with the actual time using external signals like a radio or phone time. Now, if we lack access to these external synchronization signals, the clock can significantly deviate from the correct time. Our biological clocks operate in a similar way. The biological clock can keep time, but without correction from the daily light cycle provided by the sun, the biological clock tends to lose synchrony, affecting our physical and mental well-being. A similar phenomenon occurs when we travel across time zones, resulting in what's known as "jet lag."
In the absence of natural light, our biological clocks may either gain or lose a little time. This can lead to the desynchronization of different rhythms. For example, in the absence of sufficient environmental light, the sleep-wake cycle and associated rest-activity rhythms may extend 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 believed to contribute to problems such as hormonal imbalances, sleep disorders, and mood disturbances.
Annual Rhythms
Annual rhythms, known as circannual rhythms, follow a yearly cycle and are observed in all living organisms.
Seven-Day Cycles
Circaseptan rhythms are seven-day cycles in which various biological processes, including disease symptoms and progression, resolve. Many medical professionals believe that transplant patients tend to experience more rejection episodes at intervals of seven, fourteen, twenty-one, and twenty-eight days after surgery. They also suspect that administering medications to patients at specific times may be more effective than at others, all in relation to circaseptan rhythms.
How Does the Brain Detect Light and Darkness?
Deep within the brain, specifically in the hypothalamus, two clusters of cells known as the suprachiasmatic nuclei (SCN) are located. Each SCN comprises over 8,000 neurons and serves as the body's circadian pacemaker. In mammals, the SCN appears to receive information from photoreceptors in the retina. These photoreceptors transmit signals about light and darkness through the optic nerves to the hypothalamus. Once these messages reach the SCN, a series of physiological processes are initiated.
What Occurs Once the Light/Dark Signal Reaches the SCN?
The precise mechanisms are not fully understood. The pathway from the retina, through the optic nerves to the SCN, extends further to reach the pineal gland, located near the hypothalamus above the brain stem. Upon receiving signals from the SCN, the pineal gland either secretes its primary hormone, melatonin, or inhibits the release of melatonin. This can lead to the production of serotonin, as previously explained.