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i enjoyed this article about differing views for keeping warm while you sleep. some people layer inside a sleeping bag, some go the bare minimum route, others in between. https://www.outsideonline.com/2271191/how-experts-layer-sleeping-bag
i agree with comments in the article that you need to think about keeping warm in the outdoors systemically. without adequate insulation below the bag, you will bleed a lot of heat. layering inside a bag depends on how warm or cold you sleep, how well-insulated your sleeping bag is, how much room you have inside the sleeping bag to layer without compressing the insulation, and what happens to moisture your body naturally emits while you sleep, particularly in the winter because that natural vapor you send out can freeze in the outer layers of your sleeping bag insulation, hurting loft and limiting the effectiveness of the insulation.
it is very important to me to be able to lie inside a sleeping bag when i'm shopping, for these reasons. some bags (Valandre makes a few prime examples, like the shocking blue and bloody mary) are sized to be used in combination with a good down jacket, very roomy inside, leaving space for a lofty layer inside. others, you wear a down jacket in the bag & feel like an anaconda python just swallowed you - and if the insulation is compressed, you aren't going to feel as warm inside the bag.
i have been happy using both down and synthetic fill bags in the winter, been happy at 25 to 30 below in both. a good winter bag occupies an inordinate amount of space in a backpack regardless, but the main difference was that the synthetic fill bag was truly massive and harder to compress for storage. the synthetic was good to those low temperatures, in part, because it was quite roomy, so i would sleep in a down sweater or jacket to augment the bag. the down bag is fine but not nearly as roomy, so i generally sleep in that with a light base layer, socks and a beanie. when i was taking longer trips in the cold, i used a VBL liner, slept in a light base layer.