Head Off Heat Loss
Up to 50% of heat loss in the human body occurs through the head. (That's why Mom wouldn't let you go outside without a hat when it was cold.) Similarly, a good portion of your home's heat rises straight up to your atticand your heating system has to work more to replace it.
One solution is to install an insulated, weather-stripped hatch cover at the top of the attic stairs. The stairs and the surrounding walls can also be insulated and air-sealed.