Last night was the first cool night after a hot spell and I slept soooo much better. I checked my local weather web site and it had gone down to 12°C. Perfect! I need to sleep with open windows for fresh air, even in the winter when it's snowing, and 12 degrees seems perfect to me -- well-suited to a light duvet.
10 to 15 degree Celsius would be my favorite. Summer is in full blast here, and the first time this year daytime temperature has climbed up to mid-twenties. It was pretty mild last night, high teens I would imagine, which meant I couldn�t use a duvet, but I can�t sleep without one �I will try leaving all windows open tonight!