To attempt to summarize the effetcs of magnesium is a near impossible task, however i am on a diet and my doctor, who believes in key supplements according to the needs and seasons, stresses the use of magnesium more often than any other supplement.
It's very helpful to managing stress as it's a fuel for your brain to process it, if you take it at supper it will help you sleep, and if you take vitamin D supplements in high doses (i mean the prescription kind in monthly or weekly doses), then it helps you not feel nausea.
Doctors have also concluded that our way of life and our food only provides around 300mg per day of magnesium, and we need around 400mg, so the risk of overdose is pretty much inexistant.