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Diurnal variation in body temperature, (from the 1911 Encyclopædia Britannica Eleventh Edition) ranging from about 37.5 °C from 10 a.m. to 6 p.m., and falling to about 36.3 °C from 2 a.m. to 6 a.m.

Normal human body temperature is a concept that depends on the place in the body at which the measurement is made. The value of 36.8 °C ±0.7 °C, or 98.2 °F ±1.3 °F. is the common oral measurement. Rectal measurements, or measurements taken directly inside the body cavity, are typically about a half degree Celsius (1 °F) higher. The core body temperature of an individual also tends to vary during the day and with activity level, with the lowest value in the second half of the sleep cycle; this low point, called nadir, is one of the primary markers for circadian rhythms.

In 1861, Carl Reinhold August Wunderlich claimed to measure the temperatures of one million people, and reported the average to be 37 °C.^[1]

In the United States, normal human body temperature is commonly quoted as 98.6 °F which is an exact conversion of 37 °C. This implies more accuracy than is actually present, though; to maintain the correct number of significant figures, the conversion would actually just be 99 °F.

In Russia, the commonly quoted value is 36.6 °C (97.9 °F), based on an armpit reading. According to the Russian pharmaceutical chain named 36.6, "36.6 is the ideal body temperature in Centigrade for healthy adults and children".^[2]

In reality, the expected normal core body temperature for an adult is expected to lie within a range of possible values. Although the range varies by source, one commonly cited normal range is 36.4 to 37.1 °C (97.5 to 98.8 °F). Because of the normal variation in human body temperature, this value could reasonably be rounded to 37 °C or 98 °F.

• Wong, Lena (2005). Temperature of a Healthy Human (Body Temperature). The Physics Factbook. Retrieved on 2007-08-22.

• Thermoregulation
• Carl Reinhold August Wunderlich

• Body temperature at Web MD