Stanford researchers have found that average human body temperature in the United States has steadily decreased since the 19th century.
Humans are homeothermic, maintaining a relatively constant body temperature that can shift over time. The benchmark of 37°C was set in 1851 by German physician Carl Reinhold August Wunderlich. A study published in eLife by Stanford University experts examined long-term trends to quantify these changes.
The team analyzed three datasets spanning distinct eras: medical records from over 5,000 Union Army veterans (1862–1930), a national health survey from the early 1970s, and adult patient data from Stanford Health Care (2007–2017).
Drawing from 677,423 body temperature measurements, they built a linear model to track changes over time.
The data confirmed higher temperatures in younger people, women, and those with larger builds.
Overall, men born in the 2000s have temperatures 0.59°C lower than those born in the early 1800s—a consistent 0.03°C drop per decade. Women showed a similar trend, with a 0.32°C decline per decade since the 1890s.
To rule out thermometer improvements—since thermometry was emerging in the 19th century, as lead author Julie Parsonnet notes—the researchers examined trends within each dataset using consistent measurement methods. The decline persisted, notably in the multi-decade veterans' data.
Evidence points to human body temperature cooling for at least 150 years, linked to evolving living conditions that lower metabolic rates.
Parsonnet explains: "Inflammation raises metabolism via proteins and cytokines." Public health advances—better medicine, hygiene, nutrition, and climate-controlled homes—have reduced chronic inflammation. Physiologically, we're adapted to a changed environment.
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