I am not an expert, but I understand you should have a negative uni-polar sleep pad, BUT it must be situated UNDER your mattress, NOT on top of it. This distance ensures the minor positive charge does not effect you. Bi-Polar magnets offer short-term benefits, but must not be used continually due to the positive field effects. i think Magnetico have an explanation for this or at least the man who invented their sleep pad, Dr Dean Bonlie.
A fantastic neuro surgeon called Dr Jack Kruse, who writes amazing blogs recommends the Magnetico sleep pad and uses one himself. He is certainly no fool nor has he a vested interest in them.