Make sure you dont take magnesium if you are on blood pressure meds as it will decrease your heart rate more
If one doesn't take very large doses it shouldn't be a problem.
I use propranolol and typical HR for me before bedtime is around 57-60, a few beats lower if I've taken magnesium previously. And I probably take around 150% RDI or so including what I get from food.
Make it so number one. Engage!