Masturbation.

Ejaculation causes a decrease in melatonin and may lead to insomnia. Ejaculation causes your body to kick into mating mode because your body thinks you just had sex with somebody . Your body thinks it's mating season. Your body will begin to sacrifice its hair and melatonin to increase sexual arousal. It's best to not masturbate because it is harmful and leads to an addictive downward spiral.

I've tried to accept the addiction. I've tried to fix it. But nothing works.

If you're going to ejaculate, I recommend doing so before noon so your body has enough time to recover the lost melatonin before you are ready for sleep.


http://www.huffingtonpost.com/2012/09/04/semen-good-for-womens-mental-health_n_1854074.html
According to this article, semen contains melatonin.
The problem is that there's more to this than melatonin. Masturbation increases cortisol, a wakefulness-promoting and stimulatinghormone, along with other such biohormones.

If melatonin was the only problem, a melatonin pill would fix everything, but it doesn't.