I try to keep my home office no higher than 24c/75F when I'm working. Last year I had to work in an office where it was routinely 27c/80F and above, and my productively and mood was definitely affected. As you say, above that I get sweaty and lethargic. Of course there was someone in the office complaining that even that temperature was too cold.

The weird thing is I have no issues going outside in the heat - I used to live in the middle east and would walk outside at lunch, when it was 45c/110F+ - I just can't comfortably sit at a desk and work in the heat.

A few years ago I was living and working in an apartment where in the winter it was as low as 16c/60F. I much prefer too cold than too hot. I can combat cold just by wearing more layers, especially thick socks, and drinking more tea. Does that not work for some people?