What it's normally used for
5-HTP creates serotonin, the "feel good" neurotransmitter that effects our sleep cycles, moods, and even influences our response to pain. Evidence suggests that supplementation with 5-HTP *might* promote weight loss by helping you reduce appetite and caloric intake.
5-HTP changes directly into serotonin in the brain. Serotonin has a lot of important functions, in sleep and memory (think lucid dreaming!), learning, sexual behavior, cardiovascular function, temperature regulation, appetite, mood, muscle contraction, and endocrine regulation.
What it does for Lucid Dreaming
5-HTP is one of the most popular supplements for advanced lucid dreamers, along with Galantamine and Choline.
It's the precursor to serotonin, and since it passes the blood brain barrier, it's often taken before bed to increase serotonin levels.
Serotonin helps provide deep sleep and by increasing serotonin levels, 5-HTP can help to lengthen out the deep sleep cycles during the first part of the night. Your brain then notices that it doesn't need anymore deep sleep so it then gives you what is called "REM rebound", which is extra long REM cycles during the second half of the night.
Because most lucid dreams occur during REM sleep, this makes having a lucid dream way easier because of the increased time spent in REM. This is why advanced lucid dreamers love to use 5-HTP for lucid dreaming.
5-HTP works the best if you wake up after 6 hours and take Galantamine and Choline, and it can take a week or two before getting results.
My own results
I take 5-HTP before bed. It took me almost two weeks to start getting results, and then I start getting more vivid dreams from the REM rebound and more serotonin. (This was with out taking another suppement after having six hours of sleep.)