Disclaimer: Just a bit of theorizing here, no hard facts

I've found that abstaining from ejaculation gives me more vivid dreams. If you want to see other mens' experiences, just google "no fap vivid dreams" or something of that nature.

I've also found lots of reports of vivid dreams from zinc supplementation. You can google this too.

Now, the kicker: Abstaining from ejaculation for about a week increases testosterone levels.

Zinc downregulates the 5 alpha reductase enzyme to some extent, which converts Testosterone to DHT.

So both of these methods increase T levels, and they seem to give people more vivid dreams. As we know, there's a link between androgens and visuospatial skills, and it makes sense that dreams are dependent on these skills.