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Normal human body temperature, also known as normothermia or euthermia, is a concept that depends upon the place in the body at which the measurement is made, and the time of day and level of activity of the person. There is no single number that represents a normal or healthy temperature for all people under all circumstances using any place of measurement.
Different parts of the body have different temperatures. Rectal and vaginal measurements, or measurements taken directly inside the body cavity, are typically slightly higher than oral measurements, and oral measurements are somewhat higher than skin temperature. The commonly accepted average core body temperature (taken internally) is (). The typical oral (under the tongue) measurement is , or . In Russia and former Soviet countries, the commonly quoted value is 36.6|Â°C|Â°F, based on an armpit (axillary) reading. Although some people think of these numbers as representing the normal temperature, a wide range of normal temperatures has been found. In adult men and women the normal range for oral temperature is 33.2|â€“|38.2|C|F, for rectal it is 34.4|â€“|37.8|C|F, for the Tympanic cavity it is 35.4|â€“|37.8|C|F and for axillary it is 35.5|â€“|37.0|C|F.
The time of day and other circumstances also affects the body's temperature. The core body temperature of an individual tends to have the lowest value in the second half of the sleep cycle; the lowest point, called the nadir, is one of the primary markers for circadian rhythms. The body temperature also changes when a person is hungry, sleepy, or cold.
In the early 18th century, Gabriel Fahrenheit originally used human body temperature as a reference point for his temperature scale, defining it to be 100Â°F. Later redefinition of his scale to use the boiling point of water as a reference point caused the numerical value for normal body temperature to drift.
In 1861, Carl Reinhold August Wunderlich released his summary of the armpit, or axillary, temperatures of twenty five thousand people, and reported the mean to be 37.0|Â°C|Â°F, with a range of 36.25|Â°C|Â°F to 37.5|Â°C|Â°F. He also identified the natural variations in temperature throughout the day and the variations between individuals, as well as differences based on sex and age, which were largely ignored in favor of an oversimplified single number. Wunderlich's thermometers were not calibrated to a standard settingâ€”in 1861, no standard had been agreed uponâ€”and he never explained his methods for compiling and describing the data he had collected, which would have been a monumental task before the availability of basic calculating machines. The one surviving, hand-made thermometer reads significantly higher than modern thermometers.
Temperature control (thermoregulation) is part of a homeostatic mechanism that keeps the organism at optimum operating temperature, as it affects the rate of chemical reactions. In humans the average oral temperature is 36.8|C, though it varies among individuals. However, no person always has exactly the same temperature at every moment of the day. Temperatures cycle regularly up and down through the day, as controlled by the person's circadian rhythm. The lowest temperature occurs about two hours before the person normally wakes up. Additionally, temperatures change according to activities and external factors.
Normal body temperature may differ as much as () between individuals or from day to day.
Body temperature normally fluctuates over the day, with the lowest levels around 4 a.m. and the highest in the late afternoon, between 4:00 and 6:00 p.m. (assuming the person sleeps at night and stays awake during the day). Therefore, an oral temperature of () would, strictly speaking, be normal in the afternoon but not in the morning. An individual's body temperature typically changes by about () between its highest and lowest points each day.
Body temperature is sensitive to many hormones, so women have a temperature rhythm that varies with the menstrual cycle, called a circamensal rhythm. A woman's basal body temperature rises sharply after ovulation, as estrogen production decreases and progesterone increases. Fertility awareness programs use this predictable change to identify when a woman is able to become pregnant. During the luteal phase of the menstrual cycle, both the lowest and the average temperatures are slightly higher than during other parts of the cycle. However, the amount that the temperature rises during each day is slightly lower than typical, so the highest temperature of the day is not very much higher than usual. Hormonal contraceptives both suppress the circamensal rhythm and raise the typical body temperature by about ().
Temperature also varies with the change of seasons during each year. This pattern is called a circannual rhythm. Studies of seasonal variations have produced inconsistent results. People living in different climates may have different seasonal patterns.
Increased physical fitness increases the amount of daily variation in temperature.
With increased age, both average body temperature and the amount of daily variability in the body temperature tend to decrease. Elderly patients may have a decreased ability to generate body heat during a fever, so even a somewhat elevated temperature can indicate a serious underlying causes in geriatrics.
Variations due to measurement methods
Different methods used for measuring temperature produce different results.
Generally, oral, rectal, gut, and core body temperatures, although slightly different, are well-correlated, with oral temperature being the lowest of the four.
Oral temperatures are influenced by drinking, chewing, smoking, and breathing with the mouth open
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