Normal human body temperature, also known as normothermia or euthermia, is a narrow temperature range indicating optimal health and thermoregulation. Individual body temperature depends upon the age, exertion, infection, sex, time of day, and reproductive status of the subject, the place in the body at which the measurement is made, the time of day, the subject's state of consciousness (waking or sleeping), activity level, and emotional state. Despite these factors, typical values are well established: oral (under the tongue): ±0.4 °C ( 36.8±0.72 °F), internal ( 98.2rectal, vaginal): 37.0 °C (98.6 °F).
A rectal or vaginal measurement taken directly inside the body cavity is typically slightly higher than oral measurement, and oral measurement is somewhat higher than skin measurement. Other places, such as under the arm or in the ear, produce different typical temperatures. While some people think of these averages as representing normal or ideal measurements, a wide range of temperatures has been found in healthy people. The body temperature of a healthy person varies during the day by about 0.5 °C (0.9 °F) with lower temperatures in the morning and higher temperatures in the late afternoon and evening, as the body's needs and activities change. Other circumstances also affect 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, sick, or cold.