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SleepUncovered

Melatonin — what it actually does

Updated16 May 2026Read time6 minReviewed bySleepUncovered editorial

Short answer

Moderate evidence

Melatonin is a hormone produced by the pineal gland that tells the body it's biological night. It's a timing signal, not a sedative. The mild sedation people expect is a small secondary effect; the main purpose is to coordinate the circadian phase of every clock in the body.

Key points

  • Endogenous melatonin onset (DLMO) occurs ~2 hours before habitual sleep onset.
  • For phase shifting, low doses (0.3–0.5 mg) taken 4–5 hours before DLMO produce the largest effect.
  • Higher doses (3–10 mg) don't shift phase better — and produce longer half-lives that cause morning grogginess.
  • Best evidence: jet lag, delayed sleep phase disorder, shift work. Weakest evidence: primary insomnia in adults.
  • Light exposure overrides melatonin. Bright light from a phone or screen at the time of melatonin onset can fully suppress your endogenous melatonin.

What melatonin is

Melatonin is a hormone synthesised from serotonin in the pineal gland — a small gland in the brain that responds to signals from the suprachiasmatic nucleus, your master circadian clock. Production is suppressed by light and released in darkness.

Its function is fundamentally signalling. When melatonin rises, every peripheral clock in the body — liver, gut, kidneys, immune cells — receives the message that it's biological night. Cellular metabolism shifts; repair processes activate.

DLMO — your endogenous melatonin onset

In dim light conditions, melatonin starts rising roughly 2 hours before your habitual sleep onset. This point is called DLMO(dim light melatonin onset), and it's the most reliable single marker of circadian phase.

Researchers use DLMO to study circadian shifts in lab settings. For practical purposes, DLMO is approximately 2 hours before when you naturally feel sleepy on weekends or holidays with no alarm.

What melatonin doesn't do

It doesn't knock you out. It isn't a benzodiazepine or a Z-drug. The sedation effect is small — usually a 7–10 minute reduction in sleep onset latency at typical over-the-counter doses, which most users wouldn't notice subjectively.

Most people who try melatonin and say “it doesn't work” have tested it as a sedative. The right test is as a phase-shifter, which means a different protocol entirely.

Phase-shifting protocol

The strongest effect of exogenous melatonin is shifting your circadian phase. For phase advance (sleep earlier):

  • Dose: 0.3–0.5 mg (low — yes, really).
  • Timing: ~5 hours before your target sleep onset.
  • Duration: 3–7 nights for jet lag; longer for sustained delayed sleep phase shifting.

Burgess et al (2008) mapped the human phase response curve to melatonin. The largest advancing effect occurs when melatonin is taken about 4–6 hours before DLMO — for someone with midnight sleep, that's roughly 5–7pm.

Why higher doses don't help

The melatonin receptor saturates at low doses. Going from 0.5 mg to 5 mg doesn't produce more phase shift — but does produce a longer half-life and more morning residual melatonin, which can cause grogginess and paradoxically delay the clock the next day.

Studies comparing 0.3, 0.5, 1, 3, and 5 mg consistently find that the lowest doses work as well or better for phase shifting.

Best uses

  • Jet lag (eastward especially): well-evidenced for crossings of 5+ time zones.
  • Delayed sleep phase disorder: chronic inability to fall asleep before late hours. Low-dose melatonin combined with morning bright light is the standard treatment.
  • Shift work: low-dose melatonin before daytime sleep periods for night workers.
  • Children with autism spectrum disorder and sleep issues: paediatric melatonin has good evidence in this specific population.

Weakest uses

  • Primary insomnia in healthy adults — weak evidence.
  • As a nightly sleep aid for general use — weak evidence.
  • For sleep maintenance / waking in the night — almost no evidence.

Light always wins

Bright light exposure suppresses endogenous melatonin and overrides exogenous melatonin's phase signal. If you're taking melatonin then scrolling on a bright phone for the next two hours, you've essentially cancelled the dose. Pair melatonin with dim conditions to get the effect.

Related reading

Sources

  1. 1Burgess, H. J. et al.. A three pulse phase response curve to three milligrams of melatonin in humans · Journal of Physiology · 2008PMID 18077413
  2. 2Tordjman, S. et al.. Melatonin: Pharmacology, Functions and Therapeutic Benefits · Current Neuropharmacology · 2017PMID 28503116