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What is a "normal" body temperature, anyway?

Like many kids in the U.S., I learned in science class that the temperature of the human body is 98.6 degrees Fahrenheit — a number that’s remained etched in my memory to this day. But recently, I read that a “normal” body temperature can range anywhere from 97 to 99 degrees Fahrenheit. After 20-odd years of storing “98.6 degrees Fahrenheit” in my brain, I wondered: Why, though? And WTF is my body temperature actually supposed to be?

Before tackling these burning questions (get it?), a little background on why and how your body maintains a temperature range at all: The 97 to 99 degree Fahrenheit range allows protein synthesis, metabolism and other essential processes in your body to keep running smoothly, says Christopher Moriates, an associate professor at Dell Medical School at the University of Texas at Austin. “If you get too cold or too hot, that can essentially mess up the functioning of the body.”

A region in your brain called the hypothalamus acts like a thermostat, setting your body temperature, he explains. When your body temperature increases, like it does during a workout, your hypothalamus tries to lower it by making you sweat and breathe faster. And when you get too cold, your hypothalamus tries to increase your body temperature by constricting your blood vessels and making you shiver.

So how did 98.6 degrees Fahrenheit emerge as the “normal” body temperature? Alexei Wagner, an emergency medical physician at Stanford Health Care, tells me about how German doctor Carl Wunderlich collected millions of body temperature readings from 25,000 patients in Leipzig in the mid-19th century. Their readings averaged out to 37 degrees Celsius, or 98.6 degrees Fahrenheit. And since it’s an average, “you can be on either side of it and be totally normal,” Moriates says.

But, further complicating matters, “what we’ve noticed over the years is that temperatures have really changed over time,” Wagner says. “What was normal then is not normal now.”

He cites a study, published in January, in which Stanford researchers looked at temperature readings collected at three time periods: 1862 to 1930, 1971 to 1975, and 2007 to 2017. They found that average body temperature had fallen over time. The study, and others confirming it, point to elevated baseline inflammation in the earliest readings, which might’ve raised body temperature as people’s immune systems tried to fight syphilis, tuberculosis, and other diseases common back then, Wagner says. Their increased inflammation might have also indicated uncontrolled heart disease or hunger.

Thanks to modern medicine, “we’re now a little healthier,” Wagner says. “We have people who have lower levels of inflammation that results in lower basal body temperatures.”

Besides running cooler than your ancestors, you also experience individual fluctuations in temperature, even over the course of a day. It tends to be a bit lower in the morning, when you’re less active — that is, using less energy and therefore generating less heat, Moriates explains. Then it rises a bit in the afternoon and evening, as your activity levels increase.

Should I worry if my body temperature hits 99 degrees? The simple answer: No.

Your body temperature also tends to decline as we you get older, due to drops in metabolism and muscle mass, which generate heat, Moriates says. “Older people have a harder time regulating their body temperature,” Wagner adds, explaining that distributions of cortisol and other body temperature-regulating hormones may change with age.

Menstrual cycles can affect body temperature if you have ovaries, causing it to increase by 0.5 to one degree Fahrenheit when you’re about to ovulate, Wagner explains.

Where you measure your temperature matters, too, he adds. A temperature taken from under the armpit will be a bit lower than one taken from under the tongue. And central temperatures —those taken from the rectum, for instance — are much more accurate, Moriates says.

All of this was a relief, given that my thermometer readings hover around 97 degrees Fahrenheit. But of course, other anxious thoughts crept into my head. Do I have a lower threshold for a fever? Should I worry if my body temperature hits 99 degrees?

The simple answer: No. Regardless of where you fall within the healthy range, if you have a fever driven by an infection, your temperature will exceed 100.4 degrees Fahrenheit, Moriates says. “Below that number, I wouldn’t’ worry about it.” (The COVID-19 patients he and his colleagues have seen can have 103-, 104-degree fevers.)

On its own, a fever doesn’t necessarily warrant a phone call to your doctor. “Most fevers aren’t dangerous, just uncomfortable,” which drugs like Tylenol can usually address, he says.

Moriates does suggest calling your doctor if you have a fever and you’ve recently undergone chemotherapy, have a serious underlying health condition, or experience a worrisome symptom, such as a rash, confusion, or trouble breathing. If you have a chronic condition, ask your doctor to specify the temperature that should prompt a phone call ahead of time, Wagner says.

The takeaway from my conversations with Wagner and Moriates: As long as your body temperature falls within 97 and 99 degrees Fahrenheit, you’re good. Getting different thermometer readings within that range is also totally normal, and you probably don’t need to freak out if you have a fever and nothing else.