Winter in the mid-west sucks....the moisture out of your guitar that is. The winter months, particularly January and February, can be damaging or even devastating to all wooden instruments. Cold air has less capacity to hold moisture, causing the air in your home to be dry. In addition this, forced air heat is dry if there is no humidifier in the system, further compounding the dryness in your home. This is what causes dry skin, itchy eyes, nose bleeds, etc. We are not worried about that stuff! We are worried about your instruments!
The negative effects of dryness usually manifest themselves in the form of fret poke and sinking tops first. This is when the fret board shrinks while the frets do not causing them to protrude. This leads to uncomfortable playing. The top sinks because as the top dries out, the wood contracts, pulling down the bridge leading to action that is low. This leads to a choked sounding upper register and fret buzz.
More severe or long term dryness can lead to cracks and bridges lifting. These tend to be more costly repairs and can effect the guitars structure. As mentioned before, the wood contracts and when one part moves and another doesn't, say near the pick guard or bridge, the wood will split, usually and hopefully along a grain line. The classic example is the Martin pick guard crack.
Humidity levels of 40-45% at around 70 degrees Fahrenheit are ideal conditions for an instrument. This is hard to achieve in the winter, even when running an Aprilaire home system or room humidifiers. The easiest way to maintain your instruments is to keep them in hard cases or at minimum quality gig bags with either a sound hole or case humidifier. These are readily available. We sell the Dampit, Herco, Planet Waves, and Oasis humidifiers. They all work. Some are easier to use than others but essentially, they are all some sort of moisture holding medium like a sponge or gel pack that is leak resistant and slowly evaporates inside the case creating a little happy place for your instrument.