The serotonin receptors, activated by DMT cause a decrease in serotonergic activity. It is a well known response of the body to, rather quickly increase the amount of receptors when there is a shortage of a substance or a certain type of activity. I've also read somewhere that the use of some tryptamines, although i don't exactly remember wich, maybe it was ayahuasca, cause a temporaray increase of some serotonin receptors. It may be that these receptors are increased by DMT and that these are the same receptors melatonin acts upon. It would mean that remporarily, sensitivity to melatonin is increased.