Everytime I take a melatonin supplement to help me sleep, I have nightmares all night long. If I stop taking them for a month, and then take one for a single night, guaranteed that’s the night I’ll have scary dreams. What gives? Is this a normal side effect of melatonin?
I don’t know if it’s normal, but anecdotally the one time I tried it I got nightmares so bad (on a transatlantic flight!) that I vowed never to touch the stuff again.
I don’t know but you might try 5-HTP instead
It’s a very common side effect, but it seems no one really knows why. It can cause either intense, vivid dreams or nightmares (which are, of course, intense, vivid BAD dreams.)
Google** Melatonin nightmares** and you get 128,000 hits.
Anecdotally, I’ve taken melatonin since I was 12 (I’m 22) and I’ve never once had a bad dream on them - quite the opposite, in fact, in that I usually have GOOD dreams while taking it, and when I run out I tend to have bad dreams.
But I also have a few disorders that could give me flip-flopped results from the “normal” people, so I might be the weird one. 
There are other natural supplements you can take to help you sleep - look into a few of those. I’d google “natural sleep remedies” or something like that and take it from there. Best of luck.
~Tasha
I don’t have that issue with melatonin, but I do have weird vivid dreams with valerian root. Rarely actual nightmares though.
I have very vivid dreams, but no nightmares. I’m not sure it is related to melatonin or not.
Here’s some cites:
“Very little attention has been paid to the possible side effects of melatonin. Nightmares, hypotension, sleep disorders, abdominal pain, etcetera, have been reported”
This cite seems to indicate that melatonic helps curb nitemares.
http://linkinghub.elsevier.com/retrieve/pii/S1389945703000728
There are scads of cites about dudes with "Depressed
melatonin secretion in patients with nightmares " which is just the opposite.
This cite: (pdf)
http://trainland.tripod.com/wurtman.pdf
does mention melatonin nightmares, but also notes that melatonin does work. But read it all!
This article sez that sleep quality significantly increased for the elderly.
“Sleep efficiency was significantly greater after melatonin than after placebo (83 [SE 4] vs 75 [3]%, p < 0.001) and wake time after sleep onset was significantly shorter (49 [14] vs 73 [13] min, p < 0.001). Sleep latency decreased, but not significantly (19 [5] vs 33 [7] min, p = 0.088). Total sleep time was not affected. The only adverse effects reported were two cases of pruritus, one during melatonin and one during placebo treatment; both resolved spontaneously. Melatonin deficiency may have an important role in the high frequency of insomnia among elderly people. Controlled-release melatonin replacement therapy effectively improves sleep quality in this population.”
There are something like 10000 cites with “melatonin + sleep”. Only 447 for “melatonin + nightares”.
Perhaps, you always have nitemares, but you only remember your dreams on melatonin nites? 
I suggest you discuss this with your MD.