Here is my little tip not to catch a cold while sleeping at a cold night.
Now it is getting colder in Kumamoto.(but we are having rather warm winter in Japan this year)
Every year around this season, I start using a towel like a scarf. A towel makes a big difference.
For protection against colds, when I sleep I always put a towel around my neck to keep me warm.
Only a towel helps you to keep your body warm during night,so this cold-avoiding-tip surely works.
This was also a tip for health of a Japanese famouse writer,Ryoutarou Shiba. He also kept his neck warm to avoid catching a cold.He put a scarf (or a towel. I am not sure) around his neck while he was in bed.
If you have not done yet, why not try?