Why is the average human body temperature falling by 0.05°F per decade? The short answer, is no one really knows! But Catherine Ley, a Stanford researcher who led a study to look into the curious phenomenon, has some hunches, which are laid out in this Vox article:
Healthier bodies have less inflammation, which can lead to increased body temperatures. Plus, we’ve eliminated some diseases that used to be common in the United States, and sources of fevers. “Getting rid of tuberculosis as a common infection in a population affects the overall population temperature,” Ley offers as an example. (Though the study did not directly compare average temperatures obtained in the US to areas where infections like tuberculosis are still common.)
But there could be other reasons for the change. Maybe it has to do with how we heat and cool our indoor spaces. In a well-heated home in the winter, the body may not have to work as hard to maintain a steady temperature, for instance. The proliferation of nonsteroidal anti-inflammatory drugs (like ibuprofen), which reduce inflammation and can reduce body temperature, could play a role.