Did my 5 hour one incredibly hot and humid August day in a K8 back in the mists of time. I was wearing a pair of longish shorts, trainers and a polo shirt. I managed to get away around lunchtime and went up.....and up. Finished up doing most of the flight at around 8,000' and came down like a block of ice. It took me a good half hour to get any warmth back.
Thing is what do you do? It was wiltingly hot on the ground wearing just shorts, sitting in the cockpit waiting for launch dressed for 8,000' would have been frankly dangerous. You can hardly take warm clothing with you and put it on in the confines of a glider cockpit, that's a definite non starter.