Actually THC degrades even while the plant is still alive. Because the plant is producing so much THC at peak flowering the rate of THC production far outweighs the rate of degradation. If you wait to long to chop the rate of degradation will begin to exceed the rate of production and potency will begin to decline.
When you drying your buds it's not that big of a deal to let a little light hit them, A 60 watt incandesent at 4 feet distance will be so little light that you wouldn't have to worry about it(compare that to the sun or a HID) if you need to go check on the buds for a couple minutes. however drying buds with a HID light on or in direct sunlight would be harmfull
Oxygen will also degrade THC, especially over a long period of time.