Standard melatonin backfires, causing me (and at least one other person I know) to wake up at around hour 5 after falling asleep (maybe when it has finished metabolizing). This is bad, especially if it means you cannot fall back asleep (as in my case, more than half the time).
I am sometimes physically tired by hour 16 of the day, but I can easily stay up and continue working to hour 18 or 20. I have no caffeine or nighttime sugar.
Falling asleep has been a lifelong challenge for me. I suspect there is a "preference fight" going on, among other things, keeping my thoughts highly active.