Re: How does the change in humidity affect my guitar neck?
The subject could fill a book, but in a nutshell... Materials tend to expand as they are heated and contract as they are cooled. The degree to which they react to changes in temperature depend on their Coefficent of Thermal Expansion (there are some neat materials like ceramics that can be 'tuned' to have zero CTE in a given range of temperatures, but that is another topic). Joining materials with broadly different CTE can cause them to struggle against each other (the bi-metalic strip in a thermostat, or an old blinker actually make use of this 'struggle').
A guitar, being a physical object, will tend to expand and contract with changes in temperature (truss rod and strings included and with a different CTE than the wood), but the effect is relatively small in wood, compared to changes in its moisture content. The relative humidity of a guitar's environment will also cause expansion and contraction, because of the cellular (sponge-like) composition of wood. Wood can actually contract as the temperature goes up, if the RH goes down (wood gives up moisture) and expand as the temperature goes down, if the RH goes up (wood gains moisture).
An electric has opportunities to give up moisture, even if it seems well sealed. Truss rod channels, neck pockets and pickup routs are generally unsealed. It is a good practice to keep your instrument in the 40-50% RH. Fortunately, it takes more time for 'core' moisture to transfer (compared to transfer at the surface, which is generally sealed by the guitar's finish) so transporting your guitar from one RH controlled environment to another is reasonably harmless (especially in a good case). Leaving your guitar in the trunk overnight, in the dead of a Midwest winter, not so much. If you are transporting through a temperature extreme, it is good practice to leave the guitar in its case for an hour, before getting it out. This slows the change back to room temperature/RH.
Symptoms of exposure to extremes of Temperature/RH may include changes in the action, frets loosening and exposing their ends, finish checking (spider lines), headaches, runny nose, etc.