Placebo effect may exist for someone who claims to sleep better with it, but there are effects that are definitely not placebo.

In the water that I drink in the morning I dissolve small quantities of powders of magnesium bisglycinate and potassium citrate.

Before starting to do this, after days with more intense physical effort, I frequently had nocturnal leg cramps. Since I began taking this regularly, I never had leg cramps again.