Why ‘Normal’ Body Temperatures Have Actually Decreased Over Time

Turns out, a temperature under 98 degrees is likely more accurate.

By Caroline Gellman
body heat

If you’re like me, you likely grew up thinking that 98.6 degrees Fahrenheit is the optimal body temperature. As a kid, whenever I was sick, my mom would grab the thermometer and be pleased when it had this number. But it turns out my mom may have been wrong. A new study, published in eLIFE, found that normal body temperature for men and women of all ages has decreased by about 0.054 degrees Fahrenheit each decade since the mid-19th century. 

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The researchers arrived at this conclusion after analyzing data on 190,000 body temperatures in three different eras: 1860-1940, 1971-1975, and 2007-2017. A separate study out of Harvard also highlights this change. After taking the oral temperatures of 35,000 patients, that group of researchers found the average temperature is 97.9 degrees Fahrenheit.

Essentially, this means that the average body temperature today is actually below 98 degrees Fahrenheit, so the metrics by which we all measure human body temperature is likely outdated. But then again, it isn’t so surprising considering the 98.6 degrees Fahrenheit baseline was established from data in a study conducted in 1851 by a physician who tracked the temperatures of 25,000 patients. We were definitely overdue for an update.

Why Is My Body Temperature Decreasing?

If we think back to the 1850s, people in the U.S. only lived to about 38 years. Many suffered from chronic infections like tuberculosis, syphilis, malaria, and periodontitis. In comparison, life expectancy today is about 79. While many people still suffer from chronic illness, modern medicine and all of its innovations (especially antibiotics) have helped reduce many infectious diseases that increase inflammation in the body. 

One metric associated with increased inflammation is our resting metabolic rate (RMR), or the amount of energy we expend while at rest. Because metabolism in the body creates heat as a byproduct, taking your temperature provides a rough estimate of your RMR. So, the logic is that increased inflammation will likely increase your RMR, and with it, your body temperature. For this reason, researchers point to a change in the amount of inflammation we suffer from as the main reason why our body temperatures continue to plummet.

Still, inflammation is not the only factor impacting our body temperature. The environment you live in can also impact body temperature. Maintaining our normal body temperature while the temperature around us fluctuates causes our RMR to fluctuate as well. Unlike Americans in the 19th century, many of us use heating and air conditioning in our homes. Spending less time in extreme heat or extreme cold means we spend more time in a “thermoneutral zone,” the environment at which we can maintain a normal temperature while expending the lowest amount of energy. So, expending less energy to regulate our temperatures may also account for a lower RMR and body temperature overall.

Your temperature can change throughout the day as well. If you were to take your temperature when you wake up in the morning and before you go to sleep at night, you’ll likely find that your temperature increases over the course of the day.

In reality, there is no real “normal” temperature. Women tend to have higher temperatures than men, and younger people tend to have higher temperatures than older people. Temperature also has something to do with your BMI: shorter, heavier people tend to have higher temperatures than taller, thinner people.

So, if you wonder whether that feverish feeling you had the other week with your runny nose was, in fact, an actual fever then maybe then it’s not a bad idea to figure out your baseline by taking your temperature at the same time each day for a few days and keep a record. Make sure you use the same type of thermometer each time. Oral or rectal temperatures are usually more accurate than the type of thermometer you put under your armpit.

Lastly, don’t fret too much. If you’re sick and running a fever, you’re also likely to have other symptoms that suggest you’re sick. If you listen to your body, you won’t have to rely solely on your thermometer to know something’s wrong.

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Article written by Caroline Gellman