A lot of people can have low magnesium and their only symptom is muscle cramping especially at night.  Low levels can also make blood pressure go up and cause sleeping problems. 

The best time to take magnesium is in the evening.  Start with 200-400mg and it's best to take it with calcium.  It can help with sleep, relax your muscles and lower your blood pressure.  It can also help if you are constipated.  Too much though will cause diarrhea so be careful and start with low doses.