This is gonna sound wierd, but when I took a skydiving course in the coastal area (30°C), out in the sun all day for 2 weeks, exercising, we used long pants and long sleeves, with a cotton t-shirt under the the main uniform, to catch the sweat.
Early in the morning when we got dressed, it was around 18°C. Basically, all these clothes gave us a sort of thermos effect, so you could retain a somewhat lower than ambient temperature inside your clothes. Maybe not so recommendable when you work the afternoon and night shift, since temperatures during the afternoon or in the evening might still be high.
Believe it or not, the only day we were allowed to wear shorts, it felt much hotter than with the uniform.
Maybe shorts and a cotton t shirt under a lab coat?
Definitely have enough breaks and have people drink water often. Dehydration is enough to give you terrible headaches and make you feel tired.