This will be a fairly short post. I wanted to describe techniques that work for me. This is based on no scientific knowledge whatsoever. Warm your feet at your own risk.
Why are my feet cold in the first place?
If I put my feet, which are already cold, into a cold place, then they will stay cold. I think this is because my body knows that they're cold and tries to redirect blood away from them, so that my body doesn't continue to lose internal heat to the cold place.
If my feet are already warm, and I put them into a place that is too cold, the heat will be sucked out of them, and it'll be the same situation as before.
The difference is when my feet are warm, and I put them into a warm place. The heat of my feet is able to maintain the temperature of that place, and my feet and my blankets then live in a happy symbiotic relationship of warmth for the rest of the night.
(As I said, this probably isn't scientific at all.)
My conclusion is that I need to make a warm place to put my feet in.
Um... Wear socks, Cadence?
What I said above remains true whether I wear socks or not. I do think socks help a bit, but I've had good results with and without them. I don't think they're the most important factor.
I think that the most important factor is whether the foot area is already warm before I put my feet in it.
So how do I make the foot area warm?
Your feet are too small to heat the area initially, which is why they get cold if you put them into a cold space. But a body part that is warm and large, like your legs or your butt, should be successful at initially warming the foot zone.
Here's what I do:
Before I lie down in bed, I pull back the covers and sit down on the area where my feet would go for 15-20 seconds, pulling the inside of the duvet up around my back. I find that this short period of time is enough to make the sheets and duvet comfortably warm. Then, if I lie down and put my feet into that warmed space, they never get cold again for the rest of the night.
If you find that cold air is getting in the foot end of your sheets and ruining the igloo, my final technique is to tuck the end of the duvet underneath your legs, so that your legs are surrounded on all sides and the warm air can't escape. This may require either a long duvet or a short body. If you're stuck, you could get a separate blanket to use to wrap that place.