When temperatures outdoors drop below 40 degrees farenheit, and the heat is on in your home, relative humidity can drop well below the 45 to 50% that tone woods like. Particularly in the case of acoustic instruments with all or partial solid tone woods, this can cause those woods to shrink, causing fret buzz, fret ends poking out from the fingerboard, or splits in wood.
Some of you might be using room humidifiers or even whole house humidification (i.e. April Aire). This is a great solution, as long as you monitor the humidity level, and keep it at about 45 or 50%. If you don't have an electric solution, as many of you don't, please look into the numerous ways to humidify your instrument from inside the soundhole or inside the case. These devices, which are typically a sponge or gel that is inside of a small, soft container, are inexpensive and easy to use. They range in price from $10 to about $30. You just rewet them one to three times a week (depending on how cold it is), and they keep your instrument properly humidified through the Winter months. This small investment will pay off, and protect the condition and value of your instrument.