Magnesium has been helping with this problem. a lot of times Im feeling exhausted from the heat, and then I faint. faint in the shower. iv been using magnesium oil, or spray. Im noticing I can sleep again. I am not as sensitive to the heat if I remember to take it. it was an an accidental discovery that magnesium affected my fainting. I never faint if I use it, but I started using it because of leg and arm cramps after I faint, and it stopped the fainting, and tachycardia, so I think the heat exhaustion might be connected to magnesium deficiency.