Glycine-bound magnesium has high bioavailability and minimal GI side effects. Magnesium is a cofactor in 300+ enzymatic reactions including ATP production, GABA synthesis, and muscle contraction.
Dose: 200-400 mg elemental magnesium before bed.
Caveats: Oxide and citrate forms have poor absorption / laxative effects. Threonate crosses the blood-brain barrier better but is much pricier.