Why Is My Body Temperature Fluctuating So Much?

Quick — off the top of your head, what is the normal body temperature for humans? You will probably be able to answer right away, and depending on what temperature scale you use, you'll almost certainly say it's 37˚ Celsius or 98.6˚ Fahrenheit.

This number, which you may have learned about in school, before you even started school as a worried parent noted that your temperature was a bit on the high side while you were sick, or else picked up from popular wisdom, came from a German doctor called Carl Wunderlich. Somewhere in the 1800s, Wunderlicj undertook a grand scientific test and measured the armpit temperatures of a large number of people.

The "normal temperature" we today take for granted was the average of all his measurements. Today, we know that this magic number isn't the only healthy temperature you can have — your normal armpit temperature isn't a single number, but a range. If you are aged 11 to 65, that range is 95.3 to 98.4°F (or 35.2 to 36.9°C).

Let's examine why you might be getting rather varied readings on your thermometer. 

1. The temperature on your thermometer depends on the method you use

While human core body temperature — deep inside your internal structures — remains relatively stable, it is only to be expected that your peripheral temperature, closer to your skin, is more immediately affected by external factors like the temperature in your environment, what you're currently doing and wearing, and whether you are wet or subjected to a breeze.

The fact that the temperature of your skin is so easily swayed by environmental factors is also reflected in the method you use to take your temperature. 

• Many people are used to taking their temperature by sticking a thermometer under their armpit (medically called your axillary temperature) for five minutes or so. This same method is also what those magical "normal" numbers are based on.
• If you take your temperature orally, the number you get will be around 0.55°C  or 1.0°F higher. 
• Should you take your temperature rectally, the reading will show numbers 0.27° to 0.38°C higher than the oral results, which amounts to 0.5° to 0.7°F.

It is also important to note that oral measurements will change significantly if the person drank a hot or cold beverage right beforehand, or smoked. With non-digital thermometers, even touching the thermometer with your hand will have a significant impact on the reading you end up with. 

2. Your temperature naturally fluctuates throughout the day

All humans will experience daily temperature fluctuations caused by their circadian rhythm, or internal body clock, and this is true even if they have a fever or are hypothermic (have an abnormally low body temperature). The lowest point in body temperature occurs at around 4 am for most people, while your temperature will be at its hottest point at around 6 pm. 

3. Your body temperature depends on what you do

The body's thermoregulatory and metabolic processes are constantly hard at work to maintain a balance, but nonetheless, your temperature will fluctuate with your activities and environment.

Eating produces heat, as does exercise and any other physical activity — and to cool you down, more blood will flow to the surface of your skin, where heat can dissipate. In the process, you will start sweating more profusely.

The temperature in your environment will also affect your own, cooling down when it's cold out and rising when it's hot in your environment. Interestingly, since your metabolism plays an important role in regulating your body temperature, this also means that your appetite naturally increases in colder weather, while slowing down when it's hot. 

4. Your body temperature depends on who you are

You may have heard that the core body temperature of reproductive-age women fluctuates with their menstrual cycles, being higher during the luteal phase of the cycle — the portion that stretches from ovulation to the next menstruation. Did you also know, however, that women's core body temperatures generally tend to be about 0.4 degrees Fahrenheit higher than men's? At the same time, their hands are usually, fascinatingly, slightly colder than men's. 

Infants and very young children, whose thermoregulation systems are still developing, have a slightly higher core temperature — which is also why they often have high fevers. Senior citizens, meanwhile, have been found to have slightly lower average body temperatures — their typical axillary (armpit) measurements were found to fall between 35.1 and 36.4 ° Celsius, which is 95.3 to 97.6° Fahrenheit.

5. Disease processes can cause fluctuations in your body temperature

When your core temperature exceeds 41.1° C (106° F) or drops below 32.2°C (90° F), you're in serious medical trouble. You would, however, be right to consult your doctor as soon as your thermometer indicates temperatures over 104° F (40° C) or below 95° F (35° C). 

Hyperthermia, or an excessively high temperature, has many possible causes:

• Abnormally high temperatures are usually caused by infectious processes. Most infectious will cause fevers that fluctuate on a daily basis, by as much as a degree Celsius up and down, or two degrees Fahrenheit. Some infections, like pneumococcal pneumonia and typhoid fever, do not cause significant temperature fluctuations.
• Endocrine (hormonal) disorders can like hyperthyroidism can also lead to high temperatures. 
• Extremely hot summers with high humidity levels can raise your temperature to abnormal and dangerous levels, as can hot tubs and jogging. This is one reason why it is important to wear natural fibers and light colors in summer, and to wait until after the sun goes down to engage in serious exercise.
• Even certain medications, drugs, and alcohol can lead to high temperatures, especially when combined with physical exercise. 

Hypothermia, or dangerously low body temperatures, can, meanwhile be caused by:

• Infection.
• Malnutrition or starvation.
• Bleeding in the gastrointestinal tract.
• Hormonal imbalances.
• Exposure to excessively cold environmental temperatures. 

A final word

Fluctuations in body temperature are normal — and, as such, usually not a sign that you need medical attention. Almost everything about you can impact your temperature, from your biological sex to your age, and your lifestyle to your location. Normal temperature is a range, not a single number. However, seek medical attention if your temperature measures over 104° F (40° C) or below 95° F (35° C).