I have/had fibromylgia so I find magnesium really helpful for getting a good night's sleep.

But the dosage is important. At first I was taking too little and it didn't help. Then I slowly increased to about 700 mg/day, taken before bedtime. If you take too much, you'll get diarrhea, so it's self-limiting in that respect. But be careful, a "more is better" philosophy can be deadly.