A 2001 study found a statistically significant decrease in dopamine 60 minutes after melatonin administration.

“I think that melatonin is not often thought about as impacting the dopamine pathway, but there's at least one study published in 2001 first author is Nishiyama just as it sounds. It's spelled just as it sounds. Acute effects of melatonin administration on cardiovascular autonomic regulation in healthy men. So the study wasn't specifically about dopamine, but they looked at norepinephrine and dopamine levels, and they found a statistically significant decrease in dopamine 60 minutes after melatonin administration.”