Glycine is the smallest amino acid but a major substrate for glutathione (the body's master antioxidant) and collagen synthesis, while also acting as an inhibitory neurotransmitter in the spinal cord and brainstem. Taken before bed, clinical studies show it lowers core body temperature slightly and improves subjective sleep quality and next-day cognitive performance, likely via this neurotransmitter action combined with peripheral vasodilation. It is also a popular addition to longevity stacks as a counterbalance to methionine-heavy diets, since glycine consumes one-carbon units in a way that may offset some pro-aging methionine signaling.