late to this discussion
... with my flat remodelling, I now have my bedroom at the front of the house with the kitchen, lounge and dining room being at the back and all open plan (just to put it into context, previously the front room was the lounge, and my bedroom was the bit that is now the lounge - I removed the wall between the then bedroom and dining room to make it all open plan). In said open plan I have a window and also a set of sliding doors - both south facing. Last year for the first time I bought curtains for the sliding door, not for privacy, but to keep the heat in and I do think it's made a big difference; and I also have a set of voile curtains behind them that I keep closed during the day when the main curtains are open - on colder days, they're just enough to stop the cold from the panes radiating into the room but I can still see out.
In the summer I only have the voiles to keep the temperature in the house down a bit as the sun shines right in and it can get very warm
The lounge currently has louvre blinds, but I'm saving up to replace the window and once that is done I'll have thermal conservatory style blinds fitted that clip into the actual window frames.
My bedroom is a very constant temperature throughout the year - cool in summer and warm in winter. I've not needed my electric blanket at all this year and hardly used the additional blankets - and curtains are never closed.