Indeed, cannabis can have a wide range of effects because it's not just one chemical. It's multiple cannabinoids which interact with each other.

A classic sativa variety will contain almost nothing except THC, which produces an "up" high on its own. Higher concentrations of CBD and CBN, as commonly found in indica varieties (eg, "kush" in California) and weed that has been harvested too late, cause a more "stoned" effect and sleepiness.