In all but the very warmest of Temps, i use a vry old but well made and cotton lined with eider down Brass Zipper sleping bag that has only two small Patch repairs and some new stitching on the zipper tape in my own use in the best part of fifty years. I got it in 1974 and would say it was fiftys or sixtys when produced.
Not waterproof not light not compact . not easy to dry if your unlucky enough to get it wet(Never a good idea with down).
I use and have done since the 80s British army gortex bivi bag this combo keeps you dry and super warm no matter the weather and even if your fly sheep camping.
I have a few Lighteight Synth bags , not bad for what they are and ok for summer temps, but i have had to keep gear on on occasion when using in colder temps.
I have used wool army blankets on occasion in the past, and they work to a point, and if you can build a bit of heat in the tent and its not too cold wool blankets will suffice.
Not just the thermal efficiencey but more the breathableness of bags etc, I once had a thiner down bag great for packing smallish, but it was a sort of glossy green/ blue nylon sort of outer, that was an absolute pain for condensation and ultimately WET, I hated that bag, i woke up one morning at a bike meet yet again shivering and that was it the straw that broke the camels back it was draped on the door of a skip and left, no doubt some poor sould took that thing home and had a few thourghly misserable night with that before perhaps pitching it down for the dog to sleep on perhaps.
My tip for cold in a tent or even a fly, keep youur throat well covered covered and your mouth out.