Nothing will mess up your guitar faster than low relative humidity. We're about to enter another winter (Please, NO polar vortex), and in many places, we'll be relying upon our heating systems to keep us toasty. But, while we love the blast of our 150,000BTU central heating systems, our guitars are crying out for help.
What many guitarists don’t realize is that heating our home can bring its relative humidity down to a level that can cause real harm to our six-string friends. And, sadly, the better the guitar, the more susceptible it is to drying out.
Although still vulnerable to low humidity, a solid body electric guitar with a thick poly finish and a bolt-on neck is about as bulletproof an axe you'll find. However, an acoustic guitar has thinner woods, and reacts more quickly and severely to dry air. And if the acoustic is all-solid construction (no laminates), the risk increases. Add to that, if it has a nice thin nitro finish rather than an inexpensive "shoot it & ship it" poly coating, things can turn pretty ugly.
Your guitar will thank you for keeping it in a comfortable range of 45-55% relative humidity (You can do this by humidifying the room or just humidifying the guitar). Go below that range for an extended period of time, and you're inviting cracking. One winter, I remember thinking that my "go-to" dreadnought's action was mysteriously improving. The fact is that the guitar's solid spruce top was drying out, causing the top to shrink and taking the bridge with it! Fortunately, I realized the error of my ways, and was able to reverse the situation by using a humidifier. I've had people who are convinced that their guitar has a bad neck and they show me the guitar's "hump" in the neck at the 14th fret. Almost always, this is a reaction to the top drying out and can be reversed if caught in time.
Some of us love the winter. Some of us learn to tolerate it. But if you live an an area that gets cold, you guitar hates winter. You can't really buy your guitar a pair of mittens and tell it to go outside and play. But you can easily protect it from the affects of a dry indoor winter climate.