I think the major factor is aclimatisation - when I have been on examining visits to heli units in the Middle and Far East for two week stints, I found it very difficult to cope with the feelings of fatique. After a month into a 6 month stint in Belize (+35 and vicious humidity) I realized that those feelings had gone. Apart from that, sleeping in A/C accom helps the body's core temperature to keep low(ish) and loads of water - if you don't need a pee every couple of hours, you're not drinking enough!