Serotonergic signalling is a really important regulator of sleep architecture, and this is why psychiatrists are increasingly prescribing trazadone as a sleeping aid. It was previously used as an antidepressant, but lower doses and it is a remarkably effective sleeping pill. 5-htp has similar effects, and this is why its sold as a sleep-aid supplement.

In my experience, all of these drugs can bring on intense very visually pleasant trippy dreams, with the occasional intense violent nightmare. Aside from the mood and sleep architecture effects, I wish I knew more about the neuroscience separating the positive from negative experiences.