I went Saturday 6pm to Monday morning 9am and yes it's too hot for me so made sure I started in a swim with shade and some breeze, problem was I saw fish opposite in a swim with less breeze and much less shade but had to move.
It's a long day waiting for dawn bite times and too hot to snooze in the daytime but I did have 2 takes around 8am before leaving so worth the effort.
I'm not going this weekend too hot but the weekend after I will do at least Saturday night to Sunday morning as the weather is cooling off.
If you're on a lake worth surface fishing then you could catch all day as long as you're confortable.
I have bought myself the Korda sun protection shirt (long sleeved/high neck) which means I dont have to worry about sun cream and some super lightweight combats/shorts from Fortis which were both ideal, oh and a sun hat ESP, drink lots of water.
Up at Trentview a few years back we sat in the water on our chairs most of the day to keep cool, which was lovely.
As an ex-ginger the heat isn't my thing at all and it does put me off.