By trial and error, I've discovered that it's better to close the curtains and open the window behind them, rather than close both.
If you close the window on the sunny side, the heat piles up between window and curtain and it ends up hotter than the outside. Opening the window allows that extra heat to escape.If you have a ground floor, sleep there with the back door open. That helps lose heat overnight and lets you sleep in the coolest place in the house. This entry was originally posted on Dreamwidth where it has comments.