I've had chronic sinus problems all my life. Now I've found a solution, at least partial.
Sometimes I've had particular problems breathing through my nose in bed at night. I've noticed it's been more likely to be a problem in winter. Summer has been much less of a problem, although the pattern has been quite obscured by other factors, such as colds - which for me come on more in autumn than winter - and allergies.
Recently this winter, I started wearing a woolly hat to bed at night. And it's worked a treat. I've been able to breathe through my nose most of the time at night, and my sleep has been less disrupted. This effect is definitely unrelated to viral colds, where congestion changes over the course of days, not hours.
It's easy to say that's obvious: much of our body heat is lost through the head, and since my hair is quite short I'd clearly suffer more than average in that respect.
The nose fulfils a number of biological functions - smell is only one of them. In an evolutionary sense, there is variability simply because of its function as a temperature regulator: living further away from the equator favours longer noses, to help warm the incoming air; such function is not needed by humans closer to the equator, and noses tend to be broader and flatter. That it acts as a general heat preserver does not necessarily say much about the heat exchange at a local level. Maybe for me there's a more marked local heat exchange effect than for many other people.
This is conjecture. All I can say with certainty - so far - is that I'm experiencing incontrovertable improvement in nighttime breathing through the nose by wearing a woolly hat to bed at night. The effect is somewhat less pronounced during the day - perhaps because I travel in and out of airconditioned environments frequently enough to make the effects less obvious.
So far, internet searches have turned up just a single reference to this issue - on a forum about hats. There were several anecdotes about wearing a hat easing sinus congestion. Just one of those comments mentioned medical advice to do so - by a surgeon, after a sinus operation. So the truth is out there, but not well known.
I'm keen to hear of other experiences.