I sleep colder than anyone I know. Always have. I also love to camp. And I'm allergic to wool and down. Put these three things together and you have a formula for a life-long interest: What does it take to stay warm at night? The following tips are from a lifetime of sleeping in all kinds of places.
Increase your R value. Camping teaches that you want fluffy layers near you to make spaces for warmed air. And then put a shell over them to prevent the air from floating off into the house at night. At home when it's cold I use a denim-comforter cover over a comforter, a fleece blanket, and flannel sheets.
Naked or clothed? I got to test this one out by taking a friend from California camping in October. I had two sleeping bags and she got the heavy one. I spent half the night awake in my clothes and the other half the night awake naked. The verdict? Clothes are just part of your R-value. If you have enough insulation from other places, naked can be great. But if you're cold, put on clothes. Layer if you have to. On really cold nights, I'll wear an undershirt under my flannel pj's.
Make sure you're insulated below. If you sleep in a bed, this probably isn't a problem. But if you're sleeping on the floor and the floor is a concrete slab with a layer of carpet over it then your body is bravely working to heat up the earth all night long. By sleeping on something that will warm up and stay warm you give it a break. In a pinch these days, I'd use puzzle mats. At my karate dojo, the difference between the concrete slab part of the floor and the puzzle mat part of the floor is night and day.
My cat is my hat. I learned the joys of sleeping with a night cap one week when I was living in a brick-on-brick construction house in a bedroom on the north west corner of the house. We had an early cold snap that September and didn't have the storm windows up and all the other usual winter warming things. Several nights that week I woke up to find I was sweating and my cat was wrapped around the top of my head. I figure if the cat thinks it's the warmest place in the house, I probably should add some insulation. I made a very loose-fitting watch-cap style hat from some fleece fabric and that's what I usually sleep with at night. I avoid hats that are stretchy or use elastic to stay on.
Heat your feet. My feet are my thermostat. If they're warm, I'm warm. I think there's a good reason behind this... when we're cold, our bodies turn down the blood flow to our extremities, and our feet are the most extreme of our extremities. I have a theory that there's also a feed-back loop at work... if my feet start to get cold, my body pro-actively shifts into cold mode, turning down blood flow to my feet. I learned while camping to make my water bottle with hot water and put it in the bottom of my sleeping bag. (Actually, I put in tea bags too so I have my morning tea without even turning on the stove.) At home I'll take a similar water bottle and stick it in the microwave for about 3 minutes. I did have one of those rubber water bottles for a while, but my cat poked holes in it. And if I'm chilled I'll go run my feet under warm water until the insides feel warm, put on loose-fitting socks and head back to bed.
Go to bed warm. The first job your body has when it goes to bed is heating all that insulation. So it helps to start out warm. 10 to 20 sit-ups usually does the trick for me. Also good for when you wake up cold in the middle of the night.
Make sure you're fueled and lubed. The #1 use of food energy is to keep our bodies warm. A wonderful book on camping and backpacking (and therefore on staying warm in all kinds of conditions with a very small amount of stuff) is the NOLS Wilderness Guide. They have this to say:
Fats are a more concentrated form of energy and a more complex food than carbohydrates, so it usually takes the body from two to nine hours to metabolize them. .. they are a good long-term energy source to keep you hiking all day and warm all night.NOLS Wilderness Guide, 1983, "Cooking for Nutrition and Pleasure" page 168.