It is only when the instrument is left in the cold for a long period that you need to go through an extended acclimitization. Direct sunlight is hot enough to soften the glues in your instrument and weaken or destroy the joints in the piece. Do not display your harp in your bay window unless the sun is never on that side of your home. Do not leave your stringed instrument anywhere that will be exposed to sun as the light moves across your room during the day. Never leave any stringed instrument in a car on a hot day.
Car trunks are even worse.
The simplest rule of thumb is that the same environments which are comfortable for you are appropriate for your stringed instrument.