The bottom line

Melatonin has the stronger evidence and remains the first pick for most sleep-onset problems: 0.5–3 mg taken 30–60 minutes before target bedtime reliably shortens time to fall asleep and excels at circadian problems — jet lag, shift work, delayed sleep phase; more is not better, and morning grogginess or vivid dreams are its main costs. Glycine grades preliminary but is genuinely interesting: 3 g before bed improved subjective sleep quality and next-day alertness in small randomized trials, with a plausible mechanism (peripheral vasodilation lowering core temperature, which signals sleep). Practically: if your problem is falling asleep at the wrong time, choose melatonin; if you fall asleep fine but wake unrefreshed — or want a gentler, non-hormonal option — glycine is a cheap, safe experiment (it is sweet-tasting and food-derived). They can be combined without known interaction, though the pairing is untested in trials. Chronic insomnia that resists both deserves proper evaluation — CBT-I outperforms every supplement on this page. Educational only — not medical advice; check with a clinician or pharmacist.