I'm afraid the body is not that simple. Both too little and too much mess with the heart:
> Magnesium is also cardiotoxic and, in high concentrations, can cause bradycardia [#]. Occasionally, complete heart block and cardiac arrest may occur at levels greater than 7 mmol/L. [0]
7mmol/L in just the blood supply, when magnesium supplements are in the order of 60mg per pill, would equate to an intake of about 1g of magnesium or 17 perfectly assimilated magnesium pills, in short enough time that the body has not started to eliminate the magnesium.
One to three pills before bed time is not going to pose any kind of risk.
Not true. It is low levels of magnesium that can cause heart disease. See: http://content.onlinejacc.org/article.aspx?articleid=1121693