Under the hood of every carefully selected bud is a sophisticated collection of compounds called cannabinoids. Of the dozens and dozens that we know about, THC, CBD, and CBN are the most abundant. However, this compositional structure isn’t set in stone, as each strain of cannabis will have cannabinoids in different measure. The chemical changes that take place while a cannabis plant is growing continue even after buds are harvested, stored away, and forgotten about at the bottom of a drawer.
You see, cannabinoids are quite volatile in nature, reacting to the presence of light, oxygen, and heat. If you take THC as an example, it begins life as THCA, a non-psychoactive compound that needs to be decarboxylated for users to get high. By applying heat to THCA you remove the extra carboxyl group, and the new compound, THC, reacts with the body in an entirely different way to THCA.
Expose either THCA or THC to the atmosphere or UV light for long enough, and further changes occur. This time, both compounds become CBNA and CBN, respectively. The longer you leave the weed, the higher the concentration of new compounds versus old. Although CBN isn’t bad for you, it is different, and different isn’t always what people are after.