I'll bet you have insulation in the floor of the attic but not in the ceiling part.....
The moisture from your house is getting into the attic....and the cold is causing it to condense and drip.
I remember well one night lying in bed wondering what that noise was in the attic.....it was the melted frost from the condensation dripping....
We insulated the roof part and solved the problem.
Linda C

It needs to be repair before it causes major damage and mold. I have a friend who had water in her attic and they didn't know it. In her case it was to much insulation in the attic, blown in full to the roof and they had closed all of the sofit vents thinking it would save them money. It cost her $10,000 to repair the damage to the structure and she was lucky to get it done that cheaply.