Normal body temperature is the typical temperature of a healthy human being, which is normally stated as 98.6 °F or 37 °C. However, normal body temperature is really more of a temperature range of 97.7-99.5 °F or 36.5-37.5 °C. Temperature varies according to the method of measurement, age, sex, time of day, fitness and exertion, and other factors. A temperature higher than normal is a fever, while dropping below a threshold temperature indicates hypothermia.
Methods of Measurement
The normal temperature range varies according to the site of measurement. The cited values of 98.6 °F or 37 °C are for an oral thermometer. Common measurement sites include the mouth (oral temperature), rectum (rectal temperature), armpit (axillary temperature), ear (tympanic temperature), or forehead. Rectal temperature gives the highest reading, while axillary and tympanic temperatures are the lowest.
Normal Body Temperature Is Decreasing
The temperature of 98.6 °F or 37 °C was established in the 1800s, but the average temperature is a bit lower today (97.5 °F or 36.4 °C). A study conducted by the Stanford University School of Medicine indicates normal body temperature has been decreasing at the rate of 0.003 °C per birth decade since the 19th century.
There are several possible explanations for the change, including differences in thermometers and measuring techniques, lower infection rates, lower metabolic rates relating to more sedentary lifestyles, and the prevalence of climate control in homes.
Factors That Affect Normal Body Temperature
Homeostasis maintains body temperature within a narrow range. Fluctuations within that range are natural. Several factors affect body temperature.
Circadian Rhythm
A person’s body temperature changes over the course of a day following circadian rhythms, with the lowest temperature around 4 a.m. and the highest temperature late in the afternoon between 4 pm and 6 pm (assuming a normal day/night waking and sleeping cycle). The variation between the lowest and highest temperatures is about 0.9 °F or 0.5 °C. Temperature also fluctuates from day to day by this same amount.
Hormone Cycles
Hormonal cycles also influence body temperature in both men and women, although the effect is most pronounced in menstruating women. The early morning temperature is lowest in menstruating women for the two weeks prior to ovulation. Temperature increases around 0.9 °F or 0.5 °C during ovulation and maintains this higher temperature until menstruation because the hormone progesterone is thermogenic.
Activity Level
A sedentary person tends to have a lower average temperature than an active one. Temperature rises during physical activity, too.
Age
Very young and very old people tend to have a lower normal body temperature than teens and adults and also more variability in temperature. Infants, babies, and elderly persons cannot regulate their body temperature as well as teens and adults, so temperature fluctuations are more common.
Diet
Whether you have recently eaten or not influences temperature. Metabolism increases to support digestion. So, your temperature rises slightly right after you eat a meal.
References
- Bulletin Journal (August 7, 1980). “Man’s temperature registered more than 115.7“.
- Marx, J. (2006). Rosen’s Emergency Medicine: Concepts and Clinical Practice (6th ed.). Philadelphia: Mosby/Elsevier. ISBN 978-0-323-02845-5.
- Protsiv, Myroslava; Ley, Catherine; Lankester, Joanna; Hastie, Trevor; Parsonnet, Julie (20207). Jit, Mark; Franco, Eduardo; Waalen, Jill; Rühli, Frank (eds.). “Decreasing human body temperature in the United States since the Industrial Revolution”. eLife. 9: e49555. doi:10.7554/eLife.49555
- Pryor, J.A.; Prasad, A.S. (2008). Physiotherapy for Respiratory and Cardiac Problems: Adults and Paediatrics. Elsevier Health Sciences. ISBN 978-0702039744.
- Sund-Levander, Märtha; Forsberg, Christina; Wahren, Lis Karen (2002). “Normal oral, rectal, tympanic and axillary body temperature in adult men and women: a systematic literature review”. Scandinavian Journal of Caring Sciences. 16 (2): 122–8. doi:10.1046/j.1471-6712.2002.00069.x