Recent studies suggest that the average body temperature may be lower than the traditionally accepted 98.6˚ F. An analysis of 20 studies conducted between 1935 and 1999 showed an average oral temperature of 97.5˚ F. Further supported by a 2023 study, the average body temperature is now considered to be around 97.9˚ F. This indicates that normal body temperature has been gradually decreasing over the years.

Body temperature can vary depending on the time of day, typically being higher in the afternoon than in the morning. A fever is defined as a body temperature of 100.4 F (38C) or higher. Fevers can range from low-grade, between 99.1 to 100.4 F (37.3 to 38.0 C), to high severity.

Exposure to high temperatures can lead to heat-related illnesses, with severe overheating causing symptoms like headaches, muscle cramps, and vomiting. When body temperature reaches 104 F or higher, it can lead to heatstroke, potentially damaging the kidneys, brain, and muscles.

During sleep, body temperature drops, and heart rate slows to about 20% to 30% below the resting rate, especially during deep sleep phases. High environmental temperatures can increase the risk of heart attacks, arrhythmias, and heart failure, contributing to a significant number of heat-related deaths.

The definition of fever itself varies, with medical textbooks differing in their highest normal body temperature definitions. Insomnia can also affect body temperature, with many seeking various methods to restore restful sleep.

For more detailed information on these topics, you can visit the following sources:
Time to redefine normal body temperature?,
Fever in adults,
Rising temperatures: How to avoid heat-related illnesses and deaths,
How does sleep affect your heart rate?,
Heart problems and the heat: What to know and do,
Fever – Health Encyclopedia – University of Rochester Medical Center,
Fever – Harvard Health,
Insomnia: Restoring restful sleep.