I took melatonin for weeks on end during a VERY sensitive period of time.
There was substantial insomnia during the first several months of recovery.
Considering how sensitive my serotonin system was this last year, I think I would know if there are any negative side-effects to melatonin.
I never noticed a single negative effect from the drug.
Some nights it really helped me sleep, but often it did not.
It seemed to have the greatest effect when taken on a regular basis - at least 2-3 days in a row.
Of course, lowering light exposure at night goes hand in hand.
Considering you are going to be on a rather strong stimulant and possibly exposed to lots of light, you will probably find melatonin to be lacking in effect.
But there is no risk, in my utterly cautious opinion.
And it may even offer a small amount of neuroprotection.
Valerian is a great companion to melatonin, by the way.
I think this would offer a greater effectiveness than melatonin alone.
Benedryl is NOT recommended.
I took a high dose (100mg) at 48 hrs. past my first mini-binge on MDMA.
I would swear on my own grave (and nearly did) that DPH + MDMA = Serotonin Syndrome
My best guess is that the metabolites of MDMA that were still in my system were allowed to wreak MAJOR havoc due to CYP2D6 enzyme inhibition from the high dose of DPH. This may not qualify as 'typical' SS, but SS is a poorly understood condition in the first place.
A 25mg dose of DPH following a SINGLE dose of MDMA would be very unlikely to cause such an adverse event.
But I still highly recommend against its use, esp. to 'regular' consumers of MDMA.
The shorter version of this post would have said - no problem with melatonin, but I'd add valerian to it. Don't take DPH.