Originally Posted by: Shostakovich
Can anyone explain the physics of how a guitar can tune itself up?

http://www.physicsforums.com/showthread.php?t=459222

Essentially, wood (guitar) & steel-nickel (strings) contract & expand at different rates. Overnight, both lose heat, metal faster, the guitar & strings contract, making the strings go sharp. There is also the possibility that the steel truss rod is a factor here!

When you pick it up & start playing, the strings are sharp because everything has contracted. But gradually your hands & body transfer heat to the neck & strings. But, if you retune at this point, the strings & guitar will heat up unequally, so the difference between them can make the strings go sharp a bit at first again until everything is equally "warmed up".

I usually just play for a 5-10 minutes until its warmed up & then retune. Sometimes I get impatient & tune it though. :)

This shows the importance of warming up your guitar as well as your hands! In order to avoid (as much) overnight contraction keep the guitar in a warm, stable temperature room.

Hope this helps!