- Inherent cycle of approximately 24 hours in length that appears to control or initiate various biological processes, including sleep, wakefulness, and digestive and hormonal activity.The natural signal for the circadian pattern is the change from darkness to light. The controlling mechanism for these cyclic processes within the body is thought to be the hypothalamus. Any change in the circadian cycle (such as jet lag and other conditions associated with travel) requires a certain period for readjustment.
* * *▪ biologythe cyclical 24-hour period of human biological activity.Within the circadian (24-hour) cycle, a person usually sleeps (sleep) approximately 8 hours and is awake 16. During the wakeful hours, mental and physical functions are most active and tissue cell growth increases. During sleep, voluntary muscle activities nearly disappear and there is a decrease in metabolic rate, respiration (respiration, human), heart rate, body temperature, and blood pressure. The activity of the digestive system (digestive system, human) increases during the resting period, but that of the urinary system (nervous system, human) decreases. Hormones (hormone) secreted by the body, such as the stimulant epinephrine (epinephrine and norepinephrine) (adrenaline), are released in maximal amounts about two hours before awakening so that the body is prepared for activity.The circadian cycle is controlled by a region of the brain known as the hypothalamus, which is the master centre for integrating rhythmic information and establishing sleep patterns. A part of the hypothalamus called the suprachiasmatic nucleus (SCN) receives signals about light and dark from the retina of the eye (eye, human). Upon activation by light, special photoreceptor cells (cell) in the retina transmit signals to the SCN via neurons (neuron) of the retinohypothalamic tract. The signals are further transmitted to the pineal gland, a small cone-shaped structure that is attached to the posterior end (behind the hypothalamus) of the third cerebral ventricle and that is responsible for the production of a hormone called melatonin. Cyclical fluctuations of melatonin are vital for maintaining a normal circadian rhythm. When the retina detects light, melatonin production is inhibited and wakefulness ensues; light wavelength (colour) and intensity are important factors affecting the extent to which melatonin production is inhibited. In contrast, in response to darkness, melatonin production is increased, and the body begins to prepare for sleep. Sleep-inducing reactions, such as decreases in body temperature and blood pressure, are generated when melatonin binds to receptors (receptor) in the SCN.The natural time signal for the circadian pattern is the change from darkness to light. Where daylight patterns are not consistent, as in outer space, regimented cycles are established to simulate the 24-hour day. If one tries to break the circadian rhythm by ignoring sleep for a number of days, psychological disorders begin to arise. The human body can learn to function in cycles ranging between 18 and 28 hours, but any variance greater or less than this usually causes the body to revert to a 24-hour cycle. Even in totally lighted areas such as the subpolar twilight zone, the body has regular cycles of sleep and wakefulness once the initial adjustment has been made.Any drastic shift in the circadian cycle requires a certain period for readjustment. Each individual reacts to these changes differently. Travel across a number of time zones is commonly accompanied by circadian rhythm stress, sometimes called “ jet lag.” For example, jet travel between Tokyo and New York City creates a time difference of 10 hours; it usually takes several days for the body to readjust to the new day-night pattern. Too-frequent shifts in circadian patterns, such as several transoceanic flights a month, can lead to mental and physical fatigue. Preflight or postflight adaptation can be achieved by gradually changing one's sleeping patterns to simulate those that will be necessary in the new environment. Space travel (space exploration) is even more extreme. Astronauts first encounter rapid changes in the day-night cycle while in Earth's orbit. Beyond this, the void becomes a constant blackness with no observable distinction between daytime and nighttime.The circadian cycle can alter the effectiveness of some drugs (drug). For example, the timing of administration of hormonal drugs so as to be in accord with their natural circadian production pattern seems to place less stress on the body and produce more effective medical results.
* * *
Look at other dictionaries:
Circadian rhythm — Circadian redirects here. For the album by the rock band 5th Projekt, see Circadian (album). Some features of the human circadian biological clock A circadian rhythm, popularly referred to as body clock, is an endogenously driven, roughly 24 hour … Wikipedia
circadian rhythm — circadian rhythm. См. циркадный ритм. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) … Молекулярная биология и генетика. Толковый словарь.
circadian rhythm — noun The internal body clock that regulates the (roughly) 24 hour cycle of biological processes in animals and plants. See Also: circadian … Wiktionary
circadian rhythm — Regular cycle of behaviour with a period of approximately 24 hours. In most animals the endogenous periodicity, which may be of longer or shorter duration, is entrained to 24h by environmental cues. See periodic and timeless … Dictionary of molecular biology
circadian rhythm — the regular recurrence in cycles of approximately 24 hours from one point to another, such as with certain biological activities that recur regardless of long periods of darkness or other changes in environmental conditions … Medical dictionary
circadian rhythm — (Roget s 3 Superthesaurus) n. biorhythm, cycles, body clock … English dictionary for students
circadian rhythm — /sɜkeɪdiən ˈrɪðəm/ (say serkaydeeuhn ridhuhm) noun the roughly 24 hour cycle in which physiological processes occur, some being affected by external factors such as sunlight and temperature … Australian English dictionary
circadian rhythm — noun a daily cycle of activity observed in many living organisms • Hypernyms: ↑biological time … Useful english dictionary
Circadian rhythm sleep disorder — Classification and external resources ICD 10 G47.2 ICD 9 327.3 … Wikipedia
Circadian Rhythm (film) — This article is about the thriller movie. For the biological function, see Circadian rhythm. Circadian Rhythm is a movie that focuses on a young woman and her radical journey to discover who she is and to find out why multiple enemies want her… … Wikipedia