What is normal body temperature?

Cells release heat when they convert nutrients into energy. These reactions together make up our metabolism and this is what determines our body's temperature. The body must maintain an optimum temperature to allow these chemical reactions to occur - major deviations can cause long-term organ damage.

The processes involved in maintaining a dynamic equilibrium is called thermoregulation. The hypothalamus sits at the centre of this and sends signals to lower and increase temperature depending on the situation. The body sweats when it needs to cool down as evaporation leads to a cooling of the area. The blood vessels can widen, in a process called vasodilation, to take heat from warmer parts of the body and dissipate it from the skin.

Similarly, shivering helps when you are cold as muscles generate heat by moving. Vasoconstriction takes blood away from near the surface of the body to reduce further heat loss. Your thyroid gland can also be triggered to produce more or less hormones to influence metabolism; the higher the rate of metabolism, the more heat is generated.

(Also read about cold intolerance).

Certain foods and drinks can also temporarily affect body temperature, as can exercise and other activities. 

A major study in 1992 showed that the average body temperature at 6 am was 97.5°F and 98.4°F between 4 pm and 6 pm. A study in 2018 showed that women's body termperatures were higher by about 0.2°F. This is explained in part by the fact that body temperature varies through the menstrual cycle. 

As can be seen, many factors determine body temperature and continuous bodily processes keep temperature within a range. It is, therefore, correct to think of normal temperatures to be between 97°F and 99°F and not simply as 98.6°F.

Most commonly, a temperature of over 100°F is caused by an infection. The body’s temperature rises due to the immune reaction triggered by the pathogen and not the pathogen itself. Resting and staying hydrated is important and is usually enough to clear the infection in a couple of days. Some over-the-counter (OTC) painkillers can provide respite as well.

As mentioned above, babies have a higher average range of temperature - it is between 97.9°F and 99°F. Those over the age of 65 have an average temperature less than 98.6°F. Usually, an increase of over 1°F from the average is called a fever. 

A heat stroke or an adverse reaction to drugs can also cause fever. You can also persistently have a low grade fever - this lies between 99°F and 100°F. This could be explained by hyperthyroidism or other hormonal disorders and will need to be checked by a doctor. 

Usually, if adults have fevers of over 103°F that do not break for three days, you should call a doctor. Also, if there are symptoms accompanying fever such as a rash, muscle stiffness, severe pain and disorientation, you should get medical help. 

If an infant under three months has a fever, call the doctor. For children between three months to three years with a fever of over 102°F, it is necessary to inform your pediatrician. For children older than three years, a fever higher than 103°F should be investigated.

Your body can also dip below normal. Older people generally have a lower body temperature since their metabolism slows down. Hypothyroidism, or underactive thyroid, can also lower body temperature. 

(Read more about thyroid).

Hypothermia, which is a dangerously low dip in body temperature, can be a life-threatening situation if not treated in time. In adults, a temperature below 95°F is abnormally low and is a sign of hypothermia. Often it is caused due to long exposure to extremely cold, windy and wet conditions. Hypoglycemia, shock, sepsis, anorexia nervosa and peripheral neuropathy can also lead to hypothermia. 

In babies, a temperature below 97°F is considered dangerous.

It is more helpful to think of ‘normal temperatures’ as a range from between 97°F to 99°F. There are slight variations between men and women, and babies and young children are slightly warmer than adults. Older people have a slightly lower temperature on average. 

A major study released earlier this year showed that average body temperatures have fallen by over 1°F in the last 150 years.

It is not yet known what exactly caused this decline, but those associated with the study believe that is explained in part by the fact that infectious diseases have decreased.

Infections tip body temperatures up as the immune system is continuously battling against pathogens. Note that this study was carried out using western patients, so it is unclear whether it holds true in India. However, the study does illustrate the range of normal temperatures and the many factors that influence them.

(Browse our section on infectious diseases).