I've also found that my attic cools off fairly quickly after sunset. The peak temperature occurs mid-afternoon, and begins tapering off toward sunset. It is now three hours past sundown and the attic temperature is now two degrees above ambient temperature, at 88 degrees.
Before observing this pattern I always thought attics took considerably longer after sundown to approach ambient air conditions. Perhaps more voluminous attics do, but my own attic, at least where my temperature sensor is, indicates otherwise. I may move the sensor deeper into the attic at some point to see if the pattern remains or changes.