Oddly enough, I've actually researched this topic (and the related "earthing") a lot. It's interesting because there's a surprising amount of research that seems to find positive effects. These results are reinforced by some popular "science" news (sometimes with a remarkable resemblance to pure esotericism).

However, if you dig deeper, you'll find a lot of flaws in these studies. Either suspicious financial conflicts of interest, very small sample groups, confident conclusions despite large error bars, neglect of other influences (such as being more active and outdoors while "earthing"), or just different results between similar studies.

Most studies without obvious flaws or misleading interpretations conclude that much more testing is needed before any useful statement can be made about the effectiveness of grounding/earthing.