Melatonin is the hormone your body already makes to signal that it is night. As a supplement it is one of the most-used sleep aids in North America, which makes the practical questions worth answering plainly: what does it actually do, what side effects does the research report, and who should be careful. Here is what published human evidence shows — and where it is weaker than the marketing suggests.

The short version

In a meta-analysis of 19 randomized trials (Ferracioli-Oda 2013), melatonin helped people fall asleep faster and modestly increased total sleep time and sleep quality versus placebo — but the effect was small, shortening the time to fall asleep by roughly seven minutes on average. Its evidence is strongest for sleep-onset timing and for re-aligning a shifted body clock (jet lag, shift work, delayed sleep phase), and weakest as a treatment for ongoing insomnia: the American Academy of Sleep Medicine's 2017 guideline actually gave a weak recommendation against using melatonin for chronic insomnia, citing low-quality evidence. So: a real, well-studied, but modest tool for timing sleep — not a sedative and not a cure for chronic sleeplessness.

Side effects that studies and users report

  • Next-morning grogginess or drowsiness — the most common complaint, and more likely at higher doses (this is a major reason less is often better).
  • Headache, dizziness, or nausea — reported by a minority of users.
  • Vivid or unusual dreams — commonly mentioned, generally harmless.
  • Daytime sleepiness if taken too late or at too high a dose — do not drive or operate machinery after a dose.

At typical adult doses, melatonin is generally well tolerated in short-term studies, and serious adverse events are uncommon in the published trial literature. The honest limits are the usual ones: most trials are short, so multi-year daily-use safety is not well characterized.

Who should be cautious

  • Pregnant or breastfeeding people — melatonin is essentially unstudied here; the conservative position is to avoid it unless a clinician advises otherwise.
  • Children and adolescents — this is a clinician's call, not a self-serve one. Health Canada has moved pediatric sleep-related melatonin toward prescription oversight.
  • People with autoimmune conditions or taking immunosuppressants — melatonin is immunologically active; review with a clinician first.
  • Anyone who has to be alert overnight or early the next morning — the point of melatonin is to make you sleepy at a chosen time.

Interactions worth knowing about

Because melatonin acts on the nervous system and is cleared by the liver, the interactions researchers flag are predictable. It can add to the effect of anything else that makes you sleepy, and certain medications sharply change how much melatonin ends up in your blood.

  • Sedatives, benzodiazepines, alcohol, and other CNS depressants — possible additive drowsiness.
  • Fluvoxamine and other CYP1A2 inhibitors — can raise melatonin blood levels substantially.
  • Anticoagulants and antiplatelet drugs — a theoretical bleeding-risk interaction worth a pharmacist check.
  • Blood-pressure and diabetes medication, and some contraceptives — may interact; discuss with a clinician.

Dose and timing: less is usually more

Melatonin is one of the clearest cases where a bigger number is not a better product. Low doses — often 0.5 to 1 mg — are frequently as effective as the 5 or 10 mg products on the shelf, with less next-morning grogginess. For simply falling asleep, most studied doses are taken 30 to 60 minutes before bed. For shifting a body clock (jet lag or delayed sleep phase), the timing of the dose matters more than the amount. Health Canada permits up to 10 mg per dose for adults, but starting low and only increasing if needed is the sensible approach.

How to think about it

Melatonin is a genuinely useful, well-tolerated tool for timing sleep and correcting a shifted body clock — used at a low dose, at the right time, for the right problem. It is not a sedative, and it is not the fix for chronic insomnia that some packaging implies; for that, the evidence is thin and the sleep-medicine field is lukewarm. Match the tool to the job: a small evening dose for occasional sleep-onset help or jet lag, and a clinician's input for anything persistent.

References

This article draws on the primary human research below; see the linked studies for full methods and doses.

  • Ferracioli-Oda E, Qawasmi A, Bloch MH. "Meta-analysis: melatonin for the treatment of primary sleep disorders." PLoS One, 2013;8(5):e63773. PMID: 23691095.
  • Sateia MJ, Buysse DJ, Krystal AD, Neubauer DN, Heald JL. "Clinical Practice Guideline for the Pharmacologic Treatment of Chronic Insomnia in Adults: An American Academy of Sleep Medicine Clinical Practice Guideline." Journal of Clinical Sleep Medicine, 2017;13(2):307–349. PMID: 27998379.