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 wear a balaclava at night, a thin thing that weighs nothing but works. At least I freeze less...