Common wisdom, as I’ve come to understand it, is that if you’re cold, you shouldn’t close your sleeping bag over your head as your breath condenses and you’ll get even colder. However, in my long experience of freezing at night (believe me I am very experienced), this ain’t the way it works. I’m definitely warmer inside, rather than freezing my face off through a little opening.
What say ye, condensation experts?
edit to add: Unless it’s raining heavily, I always sleep under the stars, so that may be a factor to consider. Oh and have only synthetic bags.
I too often cowboy camp or bivy camp not bothering to pitch my tarp (my bivey is a little 7 ounce thing that is really just a sleeping bag cover).
I avoid breathing in mysleeping bag or bivy if using one. I generally try very hard to avoid moisture in the bag. This includes sweat. That means not bundling up until it is cold. So I tend to progressively go warmer. Starting out not in the bag then draping it over myself, then zipping it partially, then fully, then pulling the drawstring and breathing through the little hole. I find that avoids getting sweaty in the early hours and then being cold later.
I tend to manage to stay warm with much less insulation than those around me. I don't know for sure, but suspect that one factor might be that they bundle up early and get enough sweat in their bag to be cold later. Then again I might just put out heat like a furnace.