40° internal body temperature is not the same as 40° weather.

Yucatan is not the same as Dubai in Summer.

Your body is under heat shock trying to keep up in a Sauna (that isn't considered warm until 60°). Versus a healthy body CAN keep up in 40°.

The Yucatan equivalent of a Sauna is more like doing hard labor on a roof on a sunny day with no breeze.