Quick tip: Direct sunlight is bad for your belongings
First, sunlight causes fading, not only of soft goods but also on furniture and hard surfaces. It is likely one part of your item will be noticeably faded after a season in storage. However, the result will depend on the time of year, geographic location, and how shadows are cast.
Sunlight also cause spikes in heat which can damage items. For instance, the threads in clothing, linens, and even rugs can be damage and begin to decay after prolonged exposure to sunlight.
Finally, sunlight releases moisture into the air which can increase the chance of mold and mildew.