above 20-22degC without wind chill, can just sleep naked, perhaps with a summer sheet only or silk liner (especially if there is some wind chill or temperature is dropping)
below this I add a polyester “blanket” (don't usually use a summer sheet or silk liner with this as it means more things to keep in place)
as temperature drops below 16degC, I add a woollen fire blanket (I keep this as it has many uses including bush fire protection)
if temperature drops below 14degC, I use a polyester sleeping bag liner (or a thermal fabric liner) and may add loose socks and a thin balaclava +/- thin skivvy
if temperature drops below 10degC, then I generally resort to adding a sleeping bag and definitely have a thin balaclava
if I am still feeling cold then I would add thermal leggings as well as a skivvy and loose dry socks
alternatively, I might turn on a 12V electric blanket or use a wood stove for the coldest part of the night to ensure air temperatures are maintained above 5-10degC