Probing the Mystery Behind Decreasing Human Body Temperatures

Abstract

Since 1860, human body temperature (BT) has decreased by 0.4°C from mechanisms that remain poorly understood and implications for fever ranges and population health. This study tested whether individual factors such as athletic status, BMI, sex, sleep, meals can predict BT, and used compartmental modeling to analyze what types of shifts in infection dynamics (assuming that background infections elevate temperatures) could explain the decline in BT. Cross-sectional data were collected from 101 Princeton students through a self-reported survey, oral temperature measurements in the morning and evening, and athletic classification. SIR and SIS models were solved at endemic equilibrium to estimate the parameter changes required to produce the observed BT de- cline. Both mean morning BT (36.58 ± 0.16°C) and evening BT (36.63 ± 0.17°C) were below 37°C. Among pro-athletes, BMI was negatively associated with BT (𝛽 = -0.041, p = 0.017). Only non-athletes showed a significant rise in morning-to-evening BT. SIR modeling indicated that a 2- to 6-fold increase in recovery rate could plausibly explain the observed BT decline. The SIS model also indicated a plausible mechanism through an increase in the recovery rate of 1.2 to 1.7 for acute recurrent infections (illustrative 3-7 day infections), but not for chronic infections. In both SIR and SIS models, BT decline was found to be less likely due to transmission rate changes. These findings suggest that body composition, athletic training, and reduced bacterial infection all contribute to variation in human BT. Understanding the drivers of BT decline has direct clinical importance, as fever thresholds are anchored to the old 37°C baseline and lower baseline BT have been associated with longer human life expectancies.