Biological Clock, a popular term for an internal regulator responsible for various cyclical responses in living things. Both plants and animals show yearly, monthly, daily, and other rhythmic changes that appear to be genetically programmed. Daily rhythms are called circadian rhythms (from the Latin circa dies, "about a day"). Research suggests that mammals have a biological clock that it is located in the hypothalamus of the brain, in a small area called the supra-chiasmatic nucleus.
Various circadian rhythms can be observed in people. The most obvious is the alternation of wakefulness with sleep. Disruption of the human circadian rhythm can temporarily upset the body. For example, when a passenger flies across many time zones in a few hours, the discomfort called "jet lag" results. In a few days, however, the person's biological clock adjusts to the new time.
Most cyclical responses occur at approximately the same time as changes in such external factors as light and temperature, suggesting that they are somehow caused by these external changes. Many of the seasonal changes in animals and plants are influenced by changes in day length. For instance, the decreasing amount of daylight in the fall triggers internal changes in some species of birds, causing them to migrate. Germination and flowering of plants generally occurs in the spring and summer when there is more daylight. However, experiments have shown that external factors are not always involved. For example, fiddler crabs in their natural habitat become darker in the morning, lighter in the evening. Yet when they are placed under constant environmental conditions (such as constant light) the daily rhythm of their color changes persist.
Scientists generally believe that various circadian rhythms are controlled by different mechanisms. For instance, many cycles respond to both external and internal stimuli, some cycles respond to external stimuli only, and a small number respond to internal stimuli alone.