“I recommend this (melatonin) over diphenhydramine/benadryl because that will have a dehydrating effect…” I agree, “this-that” not well stated and a bit confusing - especially if you are in need of sleep. 
What I meant is I recommend trying melatonin over the diphenhydramine. (diphenhydramine is sold under trade names like benadryl or unisom, it’s also contained in tylenol pm) Melatonin is the generic name of a naturally occurring compound.
You don’t mention your age. In my younger days, I could pretty much close my eyes, sleep lightly for 6 hrs and wake up totally refreshed. Lately it hasn’t been so easy.
A couple things. Some exercise in the early evening may help. It doesn’t have to much, maybe a brisk 15 minute walk around the block.
I use generic ambien (not CR, unfortunately). I limit myself to 3-4 times a week and must say I look forward to my “ambien nights”. When that first yawn or nod hits, I hit the ambien. A half hour later, I couldn’t continue watching the movie if I wanted to.
You might try just not pressing on with the movie after you first begin to fade. Or maybe you have?
Drink more.
Alcohol works, but I find when I really can’t sleep, I’m really stressed or dehydrated.
Big glass or 3 of water and back to bed, if I’m still awake after 20 minutes I get up, I don’t care where I have to be or what I have to do. Just get up for a bit.
If its stress I read, read and read until the book falls on my nose.
To me nothing is worse than having to be up and do something after a bad nights sleep, tossing and turning and getting aggravated that I couldn’t sleep. If I only get a few hours but made a few hundred pages in a good book, I don’t really notice that I didn’t sleep.
Another thing that works for me, when I’m doing that flippy floppy crap. I think about something I enjoy thats boring, in real time. Riding a bike, in real time in your mind, to me its enjoyable, but its boring as hell to think about, and puts me out fairly quick.
This post is one of the best short posts I’ve seen about sleep. I personally follow most of his guidelines, althoug Paul Graham (pg) is not a professional doctor/scientist/sleep researcher.