During the winter months, what was once a good level of humidity in your home might now be excessive. Cold air cannot sustain much water vapour and so condensation may start to appear on your windows as a result.
Opposite to high humidity, you could be experiencing excessively dry air instead. If cold air is coming into your house through air leaks you will experience dryness in your home. Maintaining a comfortable level of humidity inside can be a balancing act. You want the air to be moist enough to prevent sinus problems such as nosebleeds, but not too moist or else you are at risk of experiencing mould, mildew, and rotting problems over time.