You know how people tell you to drink a glass of milk before you go to bed to help you sleep? The reason for that is because milk is high in magnesium.
As one medical study puts it, "supplementation of magnesium appears to improve subjective measures of insomnia such as ISI score, sleep efficiency, sleep time and sleep onset latency, early morning awakening, and likewise, insomnia objective measures such as concentration of serum renin, melatonin, and serum cortisol."
But the quality of magnesium supplements vary widely. What I recommended is the best I've found for sleep. It's a high quality preparation that is very smooth and the dosage is quite low, only 100 mg, and a safe natural aid for sleep.