I take 600mg of magnesium a day. My blood levels were very low, and it appears I don't absorb it well .
Low levels can cause anxiety, insomnia, muscle twitching, teeth grinding, constipation and a whole host of other wonderful things. I had each one of the above.
If you are just starting do so with a small amount, 150mg would probably be a good starting point. for some people who absorb it well too much will give you "the runs" pretty quick. Work your way up to a higher level if you need to.
I take 600mg spaced out over the day to keep my levels better. I take my last dose just after supper and it is the large one at 300 mg, it really does seem to help me with insomnia. i forget to take it for a few days and I don't sleep and am backed up in the bowel department. It's a good reminder!