Humidity, or a total lack of it, can literally destroy a guitar. I knew of a guy that left his guitar out of the case and within a few feet of a radiator/heater and the dryness of air sucked all the moisture out of the guitar and the top cracked in two.
That happened OVERNIGHT. Too much mositure in the air and you're guitar will sound mushy and dull.
Temperature wise, the worst thing you can do is take an extremly cold guitar, say below 40 degrees or so, and then expose it to a warm/hot room. Ever see what happens to people that wear glasses when they come in from the cold? Their glasses get all fogged up with condensation as they are very much colder than the surrounding air. The same will happen with a guitar, so that's the same as taking a fine mist spraying atomizer and spraying your guitar, inside and out, with water. Who'd do that?