Usually when I'm ready for bed, I take one or two Benadryl and I'm out like a light dreaming pleasant dreams within a half an hour. I do take 5-htp maybe 6 or 7 hours into my evening. I've never experienced bad dreams or anything negative. By that time, I've worn myself out and it's almost 6 in the morning.
As others have posted before, I wouldn't recommend mixing the two.