My money is on the same endocrine disupters that are lowering testosterone levels, lowering sperm counts, and likely increasing obesity issues.

Here is my hypothesis. Our body temperature is expensive and evolutionary not 'stable'. In the past, it would make sense that in environments with lower bacterial threats, the group would select for lower body temperature in order to preserve energy.

In order to do that, individuals with too low a body temperature would be removed from the group due to infections. This would create a balanced system between body temperature (energy expenditure) and bacterial immunity.

With the development of antibiotics, this changed. The previous bottom of the group is not culled anymore, removing the barrier of the imbalance. Therefore, the group is 'safely' selecting for lower body temperatures.

It was a Radio Lab (Fungus Amungus): https://www.wnycstudios.org/podcasts/radiolab/articles/fungu...

The theory is that in wealthy countries with good public health, there's less need to maintain a high body temperature to fend off threats. The show said that human body temperature "has been steadily declining for decades at a rate of, like, .05 degrees Fahrenheit per decade." (though not in less developed countries)...average is said to be more like 97.5 degrees.

Interestingly in Ayurveda this is called kapha dominant body and it occurs when eats a food heavy in kapha heavy foods like sweets, dairy, cheese

So it's possible this reduction reflects a change in our diets

We are all being slowly hibernated?