However this does not mean that melatonin exerts its effects on DMT centrally (although it certainly may) I have no idea just tht it had an  effect on SWIM.  Melatonin is metabolized by CYP450 1A2 this effect may alter DMT's metabolism. when the two are combined  I have no idea if this is true although I do know that MAO activity can only account for a portion of DMT metabolism.   The point is that there are many possibilities that could lead to an enahnced reduced effect that do not involve CNS activity.