And as the trichome is exposed to air, light, and time, THCA will degrade to THC, which will then degrade to CBN, a far less psychoactive cannabinoid. By doing a potency analysis, you get the CBN numbers, and get a general sense how "fresh" the product is.
"In fact, with a high proportion of CBN, the high may start well but feels as if it never quite reaches its peak, and when coming down one feels tired or sleepy. High CBN in homegrown grass is not desirable since it represents a loss of 90 percent of the psychoactivity of its precursor THC"