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This article is about human body temperature; its common variations; and its less-common variations, such as fever. This article also briefly discusses two unusual medical conditions: hyperthermia and hypothermia.
Normal human body temperature, also known as normothermia or euthermia, 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.Despite the popular belief, 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 37 °C (98.6 °F). However, this is actually a 19th Century approximation by German Physician Dr. Carl Wunderlich. The Celsius value is set on an even degree, with the Fahrenheit merely being an overly-precise approximation. The typical oral (under the tongue) measurement, which the cited Harvard study agrees with exactly as the true average human body temperature, is slightly cooler, at 36.8° ± 0.4°C (98.2° ± 0.7°F), and temperatures taken in other places (such as under the arm or in the ear) produce different typical numbers. Although some people think of these numbers as representing the normal temperature, a wide range of temperatures has been found in healthy people.
In healthy adults, body temperature fluctuates about 0.5 °C (0.9 °F) throughout the day, with lower temperatures in the morning and higher temperatures in the late afternoon and evening, as the body's needs and activities change. The time of day and 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, or cold.
The temporal artery can also be used. It is close to the surface of the skin and therefore accessible for reading. The temporal artery is part of the main trunk of the arterial system from the heart. So long as the patient’s blood flow is permanent and regular, the method allows precise measurement of the temperature.
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 internal temperature is 37 °C (98.6 °F), 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.
In addition to varying throughout the day, normal body temperature may also differ as much as 0.5 °C (0.9 °F) from one day to the next, so that the highest or lowest temperatures on one day will not always exactly match the highest or lowest temperatures on the next day.
Normal human body temperature varies slightly from person to person and by the time of day. Consequently, each type of measurement has a range of normal temperatures. The range for normal human body temperatures, taken orally, is 36.8±0.5 °C (98.2±0.9 °F). This means that any oral temperature between 36.3 and 37.3 °C (97.3 and 99.1 °F) is likely to be normal.
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 37.3 °C (99.1 °F) would, strictly speaking, be a normal, healthy temperature in the afternoon but not in the early morning. An individual's body temperature typically changes by about 0.5 °C (0.9 °F) 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 can 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 0.6 °C (1.1 °F).
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 cause in geriatrics.
Different methods used for measuring temperature produce different results. The temperature reading depends on which part of the body is being measured. The typical daytime temperatures among healthy adults are as follows:
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 generally about 0.4 °C (0.7 °F) lower than rectal temperatures.
Oral temperatures are influenced by drinking, chewing, smoking, and breathing with the mouth open. Cold drinks or food reduce oral temperatures; hot drinks, hot food, chewing, and smoking raise oral temperatures.
Axillary (armpit), tympanic (ear), and other skin-based temperatures correlate relatively poorly with core body temperature. Tympanic measurements run higher than rectal and core body measurements, and axillary temperatures run lower. The body uses the skin as a tool to increase or decrease core body temperature, which affects the temperature of the skin. Skin-based temperatures are more variable than other measurement sites. The peak daily temperature for axillary measurements lags about three hours behind the rest of the body. Skin temperatures are also more influenced by outside factors, such as clothing and air temperature.
Many outside factors affect the measured temperature as well. "Normal" values are generally given for an otherwise healthy, non-fasting adult, dressed comfortably, indoors, in a room that is kept at a normal room temperature (22.7 to 24.4 °C or 73 to 76 °F), during the morning, but not shortly after arising from sleep. Furthermore, for oral temperatures, the subject must not have eaten, drunk, or smoked anything in at least the previous fifteen to twenty minutes, as the temperature of the food, drink, or smoke can dramatically affect the reading.
Temperature is increased after eating or drinking anything with calories. Caloric restriction, as for a weight-loss diet, reduces overall body temperature. Drinking alcohol reduces the amount of daily change, slightly lowering daytime temperatures and noticeably raising nighttime temperatures.
Exercise raises body temperatures. In adults, a noticeable increase usually requires strenuous exercise or exercise sustained over a significant time. Children develop higher temperatures with milder activities, like playing.
Psychological factors also influence body temperature: a very excited person often has an elevated temperature.
Sleep disturbances also affect temperatures. Normally, body temperature drops significantly at a person's normal bedtime and throughout the night. Short-term sleep deprivation produces a higher temperature at night than normal, but long-term sleep deprivation appears to reduce temperatures. Insomnia and poor sleep quality are associated with smaller and later drops in body temperature. Similarly, waking up unusually early, sleeping in, jet lag and changes to shift work schedules may affect body temperature.
A temperature setpoint is the level at which the body attempts to maintain its temperature. When the setpoint is raised, the result is a fever. Most fevers are caused by infectious disease and can be lowered, if desired, with antipyretic medications.
An early morning temperature higher than 37.2 °C (> 98.9°F) or a late afternoon temperature higher than 37.7 °C (> 99.9°F) is normally considered a fever, assuming that the temperature is elevated due to a change in the hypothalamus's setpoint. Lower thresholds are sometimes appropriate for elderly people. The normal daily temperature variation is typically 0.5 °C (0.9°F), but can be greater among people recovering from a fever.
An organism at optimum temperature is considered afebrile or apyrexic, meaning "without fever". If temperature is raised, but the setpoint is not raised, then the result is hyperthermia.
Hyperthermia occurs when the body produces or absorbs more heat than it can dissipate. It is usually caused by prolonged exposure to high temperatures. The heat-regulating mechanisms of the body eventually become overwhelmed and unable to deal effectively with the heat, causing the body temperature to climb uncontrollably. Hyperthermia at or above about 40 °C (104 °F) is a life-threatening medical emergency that requires immediate treatment. Common symptoms include headache, confusion, and fatigue. If sweating has resulted in dehydration, then the affected person may have dry, red skin.
In a medical setting, mild hyperthermia is commonly called heat exhaustion or heat prostration; severe hyperthermia is called heat stroke. Heat stroke may come on suddenly, but it usually follows the untreated milder stages. Treatment involves cooling and rehydrating the body; fever-reducing drugs are useless for this condition. This may be done through moving out of direct sunlight to a cooler and shaded environment, drinking water, removing clothing that might keep heat close to the body, or sitting in front of a fan. Bathing in tepid or cool water, or even just washing the face and other exposed areas of the skin, can be helpful.
With fever, the body's core temperature rises to a higher temperature through the action of the part of the brain that controls the body temperature; with hyperthermia, the body temperature is raised without the consent of the heat control centers.
In hypothermia, body temperature drops below that required for normal metabolism and bodily functions. In humans, this is usually due to excessive exposure to cold air or water, but it can be deliberately induced as a medical treatment. Symptoms usually appear when the body's core temperature drops by 1-2 °C (1.8-3.6 °F) below normal temperature.
Basal body temperature is the lowest temperature attained by the body during rest (usually during sleep). It is generally measured immediately after awakening and before any physical activity has been undertaken, although the temperature measured at that time is somewhat higher than the true basal body temperature. In women, temperature differs at various points in the menstrual cycle, and this can be used in the long-term to track ovulation both for the purpose of aiding conception or avoiding pregnancy. This process is called fertility awareness.
Core temperature, also called core body temperature, is the operating temperature of an organism, specifically in deep structures of the body such as the liver, in comparison to temperatures of peripheral tissues. Core temperature is normally maintained within a narrow range so that essential enzymatic reactions can occur. Significant core temperature elevation (hyperthermia) or depression (hypothermia) that is prolonged for more than a brief period of time is incompatible with human life.
Temperature examination in the rectum is the traditional gold standard measurement used to estimate core temperature (oral temperature is affected by hot or cold drinks and mouth-breathing). Rectal temperature is expected to be approximately one Fahrenheit degree higher than an oral temperature taken on the same person at the same time. Ear thermometers measure eardrum temperature using infrared sensors. The blood supply to the tympanic membrane is shared with the brain. However, this method of measuring body temperature is not as accurate as rectal measurement and has a low sensitivity for fevers, missing three or four out of every ten fevers in children. Ear temperature measurement may be acceptable for observing trends in body temperature but is less useful in consistently identifying fevers.
Until recently, direct measurement of core body temperature required surgical insertion of a probe, so a variety of indirect methods have commonly been used. The rectal or vaginal temperature is generally considered to give the most accurate assessment of core body temperature, particularly in hypothermia. In the early 2000s, ingestible thermistors in capsule form were produced, allowing the temperature inside the digestive tract to be transmitted to an external receiver; one study found that these were comparable in accuracy to rectal temperature measurement.
Fevers are not to be confused with heat stroke. In fever the person can feel cold at high body temperatures. One theory is that the body is fooled into thinking it is cold by the infectant microbe. The more recent alternative hypothesis is that fever is a constructive response to infection, and that the chills are an evolutionary mechanism whose function is to motivate individuals to seek warmth, to help facilitate the increase in body temperature.
There is a risk of injury from cracking glass thermometers if too much force is applied by the teeth to hold them in place, and the alcohol or mercury contents are poisonous. These older thermometers are gradually being replaced by electronic thermometers which are made from solid plastic and use a metal (thermocouple) sensor.
A plastic thermometer strip placed on the forehead gives an approximate local reading, which depends to a great extent on ambient air temperature and local circulation effects. Using a thermometer to record the temperature under the armpit is less affected by surrounding air temperature, but is still prone to diverge from true core temperature if there are alterations in blood circulation.
Since 2000, small ear thermometers have become available. It is thought that the eardrum closely mirrors core temperature values, and these devices work by detecting the infrared heat emission from the tympanic membrane. A measurement is quickly taken within one second, making them popular for use with children. While the electronic display of the temperature value is easier to read than interpreting the graduation marks on a thermometer, there are some concerns about the accuracy of ear thermometers in home use.