Brief Answer:
Taking 20 mg of melatonin about an hour before NyQuil is safe

Detailed Answer:
Hi XXXX,
Thanks for writing in to us.

I have read through your query in detail.
After having reviewed latest medical database, it is confirmed that melatonin and NyQuil (acetaminophen / dextromethorphan / doxylamine / pseudoephedrine) do not interact with each other in any way when taken at recommended doses.

It means that you will not experience any adverse effect due to taking the medications as described by you. However, should you have any discomfort after taking these medications then please consult your doctor immediately.

Hope this answers your question. Please feel free to correct any oversight in my interpretation of your problems and discuss them in detail as per your requirements.

Hope your query is answered.
Do write back if you have any doubts.

Regards,
Dr.Vivek

---