If somebody is "manifesting" themselves a sleep aid, I think they'd just call it meditation and everybody would more or less accept that it probably works for that individual. Maybe you'd have a few people with severe autism who start arguing on online forums about the scientific evidence behind meditation, but that's just them being them.

The placebo effect is a real, measurable mind-body response where belief & expectation can change your symptoms or how you feel. However, it does not directly alter external reality. Manifesting claims your thoughts or intentions can cause _outside events_ to happen, which has zero evidence to support it.