![]() ![]() The first 10 days (1–10) and last 10 days (35–45) were spent in natural conditions outside the cave, and therefore his activity was synchronized to a 24-h clock. The thick lines symbolize the time Siffre spent asleep. ![]() The daily activity from the beginning of the experiment (top) to the end (bottom).In normal conditions, when we are exposed to sunlight, our clock cycle shortens to exactly 24 h. This clock creates daily rhythms, with a cycle slightly longer than 24 h, even in the absence of environmental cues. The main conclusion was that the body has an internal clock, which is independent of the environment’s fluctuations. In fact, the length of the “day” he lived by in the cave was 24.5 h. The results of the experiment ( Figure 1) showed a daily, cyclical, organized activity pattern, where the significant difference was that each day he woke up about half an hour later. He lived in a tent with an artificial lighting, connected to machines that tracked his body’s activity. He lived for 2 months in a cave in the Alps, without exposure to daylight, with constant temperature, and without knowing if it was day or night. How can we differentiate between these two mechanisms? In 1962, a French scientist named Michel Siffre conducted an experiment on himself. An additional explanation is that there is an internal mechanism in the body, creating the daily rhythm independently of environmental changes. An increase in light levels with sunrise makes us wake up, and the darkness at night results in an increase in melatonin, which promotes sleep. What drives the daily rhythms in our bodies? One possibility is that the body responds to cyclical changes in the environment. The Biological Clock Creates Daily Rhythms We are also all aware of daily cycles in our emotional and behavioral processes, such as our alertness and our ability to concentrate or learn, and the cycle of wakefulness and activity during the day and sleep during the night. The concentrations of many other proteins in our bodies also show daily fluctuations. For example, the secretion of a hormone called melatonin (the sleep hormone) reaches its peak late at night and decreases in the morning, whereas, in the morning, the hormone cortisol reaches its peak. Many processes in our bodies show daily fluctuations, including our body temperature, blood pressure, and hormone levels. So, it is not surprising that, during evolution, the daily cycle was a significant factor to which animals and plants adapted. This cycle results in environmental changes during the day, such as higher light levels and warmer temperatures. The earth’s rotation creates the phenomenon of day and night in a 24-h cycle. Repeating processes, such as waves moving up and down, create a rhythm characterized by a consistent cycle. The study of chronobiology studies the mechanisms of the biological clock and the clock’s influence on human health. The modern world has created disruptions in the circadian clock’s timing, because of electrical lighting, flights to other time zones, and work during the night. The biological clock prepares the body for environmental changes. The nervous system transfers information about the external light level to the biological clock in the brain, which matches the clock’s cycle to the external environment. Even in the absence of changes in light between day and night, the biological clock creates cycles called circadian rhythms. During evolution, plants and animals adapted to these cycles, developing daily cycles of physical and behavioral processes that are driven by a central biological clock, also known as the circadian clock. Earth’s rotation creates a cycle of day and night, which is observed as changes in light levels and temperature. ![]()
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