time when you come down from the high and when you hit the bed... that'll augment its effectiveness.
despite common belief, marijuana is a stimulant. as you come down off a stimulant you get tired. it has some other overlapping effects obviously, but bottom line is your heart and metabolism speed up temporarily when high and when that effect wears off you feel comparably tired to before.
as with sleeping pills, depending on pot to sleep is not a healthy long-term solution. not sure if as useless as drunk sleep, but possibly not as restful too. Conditioning a dependence on it isn't wise.