In regards to LDs, the placebo effect refers to the idea that believing something will cause a LD, and will generally do so; dreams and sleep are generally more susceptible to belief, expectations, autosuggestion and whatnot, so it's a fairly noticeable benefit.

Example, over in the Lucid Aids forum there's a thread about B6. One guy there posted a few links to studies that show that B6 is actually more likely to reduce REM sleep and such. But people around here still insist that it's the best thing evarz in helping with LDs.

There's a difference between autosuggestion and placebo.