You wake up at 6 am feeling achy and chilled. Unsure if you’re sick or just sleep-deprived, you reach for a thermometer. It beeps at 99°F, so you groan and roll out of bed and get ready for work. Because that’s not a fever. Is it?

Yes, it is. Forget everything you know about normal body temperature and fever, starting with 98.6. That’s an antiquated number based on a flawed study from 1868 (yes, 150 years ago). The facts about fever are a lot more complicated.

First, there’s no single number for normal. It’s slightly higher for women than men. It’s higher for children than adults. And it is lowest in the morning.

"A temperature of 99 at six o’clock in the morning is very abnormal, whereas that same temperature at four o’clock in the afternoon can be totally normal," says Jonathan Hausmann, a rheumatologist at Boston Children’s Hospital and Beth Israel Deaconess Medical Center in Boston, who gathered 11,458 temperatures in crowdsourced research using an iPhone app called Feverprints.

The study, published online this month in the Journal of General Internal Medicine, refutes the age-old benchmark of 98.6 degrees Fahrenheit. Instead, Hausmann and his colleagues found an average normal temperature in adults of 97.7 degrees, as measured with an oral thermometer. (The published study uses results from 329 healthy adults.) As for fever, Hausmann found that it begins at 99.5 degrees, on average.

But that doesn’t mean you should shift to a lower benchmark for normal. Hausmann wants body temperature to be a flexible concept, viewed in context with age, gender, time of day, and other factors—much in the way weight is evaluated based on height, and how the thresholds for normal blood pressure differ based on age.

Hausmann isn’t the first to push back on the definition of normal body temperature, but it’s been hard to topple 98.6 degrees. It remains on major medical websites. As for fever, the nation’s public health agency, the Centers for Disease Control and Prevention, puts 100.4 degrees at the lower bound. But in a nod to fever's slippery nature, the CDC notes that feeling warm to the touch or "feverish" is also sufficient.

So if you think you have a fever, you probably do.

Our internal thermostat lies in the hypothalamus, an almond-sized area of the brain that induces us to sweat when we need to cool down and shiver when we need to warm up. Body temperature rises with exercise, in hot weather, and after taking some types of drugs, including some antibiotics and antihistamines. Women also run higher temperatures during ovulation and pregnancy.

In the body’s first response to pathogens, proteins called pyrogens flow through the bloodstream to the hypothalamus, which responds by ramping up the heat. Fever helps your body fight infection by stimulating the immune system, sending a kind of alert to the body’s defenses. It also creates a more hostile environment for bacteria and viruses, making it more difficult for them to replicate. Though parents often worry when their young children spike a fever, a high temperature is the vanguard, not the enemy.

Unless a patient’s temperature is 103 degrees or higher, family physician Leonard Reeves typically doesn’t advocate treating fever. "Your body is going to try to maintain an elevated core temperature no matter what you do," says Reeves, who is on the board of directors of the American Academy of Family Physicians and practices in Rome, Georgia. "It’s best to find the source of the infection and fight that."