The post-MDMA drop in energy or fatigue is likely due to a compensatory surge in prolactin following a dopamine surge.

“I spoke to a physician colleague of ours who said that that very likely has something to do with the surge in prolactin that arrives subsequent to the big dopamine surge that occurs in MDMA. And I mention that because I know a number of people talk about serotonin depletion after taking MDMA. He has it in mind that while that could be true, it's likely that anytime somebody takes something or does something where there's a huge lift in dopamine, that there's very likely a huge compensatory increase in prolactin that follows and prolactin has a kind of sedative effect, numbing effect on mood and libido, et cetera, that eventually also wears off.”