Hot Facts! Five Things You Didn’t Know About Body Temperature

If you’ve lived on earth this past year you’ve had your temperature taken. Ever wonder about the number that came up on the thermometer? Read on to find out some interesting facts about our temperatures.

• You may have heard that a ‘normal’ temperature is 37°C/98.5°F. Did you know that this number originally comes from a nineteenth-century medical study? In the 1860s a German doctor by the name of Carl Wunderlich took ~25,000 human temperatures and got 37oC as an average. This was no small feat as the early thermometer he used was about a foot long and took around twenty minutes to settle on a number! Dr. Wunderlich is also attributed with introducing patient temperature charts to his university’s hospital and promoting the idea that fever was a symptom of multiple diseases rather than a disease itself.

• Temperatures vary widely within humans. Age, physical fitness, and genetics all influence a person’s temperature. Consequently, a temperature between 33.2–38.2°C/91.8–100.8°F could be considered ‘normal’. For example, babies and children tend to run a little higher (35.5–37.5°C/95.9–99.5°F) and older adults run lower. Interestingly, older adults not only have a lower average healthy temperature but also a decreased ability to generate a fever during mild infections. More serious viral and bacterial infections still trigger fever across the age range so temperature screenings for fever above 38°C/100.4°F are still a useful detection method.

• Temperatures also vary within an individual. It cycles regularly throughout the day as part of our natural circadian rhythm. We generally hit our lowest temperature during the second half of our sleep cycle (between 2 and 4 am) and our highest temperature at the end of the day (between 4 and 6 pm). These two temperatures can differ by as much as 0.5°F. An individual’s temperature also fluctuates with exercise, hormones, food/water consumption, and even mood.

• Average human body temperatures seem to be decreasing. A team of Stanford scientists compared temperature data sets from three periods: 1860-1940, 1971-1975, and 2007-2017 and were able to normalize for factors like differences in temperature taking methods. They found a significant decrease in healthy body temperatures over time across several different groups. For example, they found the temperature of men born in the early to mid-1990s was 1.06°F lower than that of men born in the early 1800s. Why are our bodies cooling down? One hypothesis is that our temperature is determined by the dual pressures of conserving calories and protecting against infection – as medical treatments, hygiene standards, and food safety have increased we’ve been able to safely lower our average body temperatures.

• Most warm-blooded animals seem to have settled on a similar temperature range. Normal temperatures for mammals are generally between 36.1-39.4°C/97-103°F. One notable exception – camels can fluctuate anywhere between 33.9-41.7°C /93-107°F. Birds tend to run a little warmer with a temperature range of 37.7-43.5°C/99.9-110.3°F. Bird temperatures also fluctuate more. Within 24 hours a robin’s temperature can fluctuate 6 °C (11 °F) while a house wren’s temperature can change as much as 8 °C (14 °F).

Teaching thermoregulation this semester? There are a ton of great resources out there including this Nature Education article and it’s sources, these homeostasis lesson plans, or this Origami Organelles download.