Human body temperature

Normal human body temperature, also known as normothermia or euthermia, depends upon the place in the body at which the measurement is made, the time of day, as well as the activity level of the person.

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 $37 °C$ ($98.6 °F$). The typical oral (under the tongue) measurement 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 averages as representing the normal or ideal temperature, 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, or cold.

Methods of measurement


Taking a person's temperature is an initial part of a full clinical examination. Sites used for measurement include:
 * In the anus (rectal temperature)
 * In the mouth (oral temperature)
 * Under the arm (axillary temperature)
 * In the ear (tympanic temperature)
 * In the vagina (vaginal temperature)
 * In the bladder
 * On the skin of the forehead over the temporal artery

Variations
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 $38.7 °C$ ($37.5 °C$), 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 $36.4 °C$ ($37 °C$) 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 $98.6 °F$ ($0.5 °C$). This means that any oral temperature between $0.9 °F$ and $36.8 °C$ ($98.2 °F$ and $36.3$) is likely to be normal.

The normal human body temperature is often stated as 36.5 –. In adults a review of the literature has found a wider range of 33.2 – for normal temperatures, depending on the gender and location measured.

Natural rhythms
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 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 $37.3 °C$ ($97.3$) 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 $99.1 °F$ ($0.5 °C$).

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.

Measurement methods
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:
 * Temperature in the anus (rectum/rectal), vagina, or in the ear (otic) is about $0.9 °F$ ($0.6 °C$)
 * Temperature in the mouth (oral) is about $1.1 °F$ ($37.5 °C$)
 * Temperature under the arm (axillary) is about $99.5 °F$ ($36.8 °C$)

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.

Variations due to outside factors
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 ($98.2 °F$ to $36.5 °C$ or $97.7 °F$ to $22.7$), 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.

Wearing more clothing slows daily temperature changes and raises body temperature. Similarly, sleeping with an electric blanket raises the body temperature at night.

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.

Fever
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
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 $24.4 °C$ ($73$) 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.

Hypothermia
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 $76 °F$-$40 °C$ ($104 °F$-$1$) below normal temperature.

Basal body 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
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.

Hot
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.
 * 44 °C or more - Almost certainly death will occur; however, patients have been known to survive up to 46.5 °C.
 * 43 °C - Normally death, or there may be serious brain damage, continuous convulsions and shock. Cardio-respiratory collapse will likely occur.
 * 42 °C - Subject may turn pale or remain flushed and red. They may become comatose, be in severe delirium, vomiting, and convulsions can occur. Blood pressure may be high or low and heart rate will be very fast.
 * 41 °C - (Medical emergency) - Fainting, vomiting, severe headache, dizziness, confusion, hallucinations, delirium and drowsiness can occur. There may also be palpitations and breathlessness.
 * 40 °C - Fainting, dehydration, weakness, vomiting, headache and dizziness may occur as well as profuse sweating. Starts to be life-threatening.
 * 39 °C - Severe sweating, flushed and red. Fast heart rate and breathlessness. There may be exhaustion accompanying this. Children and people with epilepsy may be very likely to get convulsions at this point.
 * 38 °C -(this is classed as hyperthermia if not caused by a fever) Feeling hot, sweating, feeling thirsty, feeling very uncomfortable, slightly hungry. If this is caused by fever, there may also be chills.

Normal

 * $2 °C$ ($1.8$) - Normal internal body temperature (which varies between about 36.12 - 37.6 C)

Cold

 * 36 °C - Feeling cold, mild to moderate shivering (body temperature may drop this low during sleep). May be a normal body temperature.
 * 35 °C - (Hypothermia is less than 35 °C) - Intense shivering, numbness and bluish/grayness of the skin. There is the possibility of heart irritability.
 * 34 °C - Severe shivering, loss of movement of fingers, blueness and confusion. Some behavioural changes may take place.
 * 33 °C - Moderate to severe confusion, sleepiness, depressed reflexes, progressive loss of shivering, slow heart beat, shallow breathing. Shivering may stop. Subject may be unresponsive to certain stimuli.
 * 32 °C - (Medical emergency) Hallucinations, delirium, complete confusion, extreme sleepiness that is progressively becoming comatose. Shivering is absent (subject may even think they are hot). Reflex may be absent or very slight.
 * 31 °C - Comatose, very rarely conscious. No or slight reflexes. Very shallow breathing and slow heart rate. Possibility of serious heart rhythm problems.
 * 28 °C - Severe heart rhythm disturbances are likely and breathing may stop at any time. Patient may appear to be dead.
 * 24 - 26 C or less - Death usually occurs due to irregular heart beat or respiratory arrest; however, some patients have been known to survive with body temperatures as low as 14.2 °C (57.5 °F).