Melatonin is the hormone secreted by the pineal gland in response to darkness. Falls in production with age.
Mechanism — Binds MT1/MT2 receptors to signal night to the brain and body. Lowers core temperature, gates sleep onset, modulates the master clock in the suprachiasmatic nucleus. Also a potent direct antioxidant in mitochondria.
Lab range — Salivary or 24-hour urine measurements used in chronobiology research. Not routinely tested clinically. Supplemental dose: 0.3-1 mg sublingually 30-60 min before bed for sleep; higher doses (3-10 mg) for antioxidant/longevity use.
Caveats — Most US supplements are over-dosed 5-10x. More is not better for sleep; higher doses can disrupt morning cognition. Cycle awareness needed — modulates other hormones at high chronic doses.