Normal human body temperature
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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 body. Although the value 37.0 °C (96.6 °F) is the commonly accepted average core body temperature, the value of 36.8±0.7 °C, or 96.6±1.3 °F is an average oral (under the tongue) measurement. Rectal measurements, or measurements taken directly inside the body cavity, are typically slightly higher. In Russia and former Soviet countries, the commonly quoted value is 36.6 °C (97.9 °F), based on an armpit reading. 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.
Gabriel Fahrenheit originally used human body temperature as a reference point for his temperature scale, defining it to be 96°. 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 data on the armpit, or axillary, temperatures of twenty five thousand people, and reported the mean to be 37.0 °C (98.6 °F), with a range of 36.2 °C (97.2 °F) to 37.5 °C (99.5 °F).
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 (98.2 °F), though it varies among individuals, as well as cycling regularly through the day, as controlled by one's circadian rhythms with the lowest temperature occurring about two hours before one normally wakes up.
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 37.2 °C (99.0 °F) would, strictly speaking, be normal in the afternoon but not in the morning. An individual's body temperature typically changes by about 0.5 °C (0.9 °F) between its highest and lowest points each day.