My son spent two years in the Philippines as a missionary for the Church of the Latter-Day Saints. One of the things that made the biggest impact on him while there was the intense heat.
I knew the heat could be intense in the Philippines, but I didn't really understand it until my son explained how it affected him. 
So when your OFS asks to adjust their work schedule to accommodate the heat, let them.  It does affect their productivity and their health. It's one of those cultural things you should know when working with an OFS.