Normal body temperature is 98.6 degrees Fahrenheit, right? Not so. There is no baseline for humans, and even if there was, it would be closer to 97.7 °F. Temperature also varies across the day, peaking in late afternoon and bottoming out in early morning. It is slightly higher for women than for men as well. For two decades research has debunked the benchmark, set way back in 1868, yet it persists. One important ramification, says Jonathan S. Hausmann, a rheumatologist at Boston Children's Hospital, who led the latest study, is to redefine fever. Most doctors use 100.4 °F or higher, but if “normal” is lower, then the fever threshold should be, too. It also should vary with the daily pattern and be tailored to each individual, Hausmann says: “A child at 99.0 °F at 4 A.M. may be highly abnormal but at 4 P.M. could be within normal limits.”

Credit: Nadieh Bremer; Sources: “Normal Oral, Rectal, Tympanic and Axillary Body Temperature in Adult Men and Women: A Systematic Literature Review,” by Märtha Sundlevander et al., in Scandinavian Journal of Caring Sciences, Vol. 16, No. 2; June 2002; “A Critical Appraisal of 98.6 °F, the Upper Limit of the Normal Body Temperature, and Other Legacies of Carl Reinhold August Wunderlich,” by Philip A. Mackowiak et al., in JAMA, Vol. 268, No. 12; September 23–30, 1992; “Using Smartphone Crowdsourcing to Redefine Normal and Febrile Temperatures in Adults: Results from the Feverprints Study,” by Jonathan S. Hausmann et al., in Journal of General Internal Medicine. Published online August 13, 2018