Short answer
Melatonin is a hormone released by the pineal gland at night, when it is dark, and switched off by light. It does not knock you out; it signals “biological night” to the body and marks the timing of your internal clock. Its natural evening rise is one of the most reliable markers of circadian phase.
Simple explanation
Melatonin is sometimes called the “hormone of darkness.” As evening falls and light fades, your brain’s master clock tells the pineal gland to start releasing it. Levels climb through the night and drop again toward morning. In effect, melatonin is your body’s internal announcement that night has arrived – a message that helps sleep along and keeps the rest of your daily rhythm anchored.
An important point often misunderstood: melatonin is more of a timing signal than a sedative. It tells your body when it is night rather than forcing you unconscious. That is why its main value against jet lag comes from when it is present, not from how strong the dose is.
Deep dive
The release schedule is controlled by the suprachiasmatic nucleus, which drives the pineal gland via a dedicated pathway. Because light suppresses melatonin, evening exposure to bright, blue-enriched light can blunt or delay the natural rise – one of the ways late-night screens can nudge the clock later.
The point in the evening when melatonin starts to climb, measured under dim conditions, is called the dim light melatonin onset (DLMO). It usually occurs about two to three hours before habitual sleep and is treated by researchers as a gold-standard marker of a person’s circadian phase – a way of reading what time the internal clock currently thinks it is.
Melatonin also has its own phase response curve, and it is roughly the mirror image of the one for light. In broad terms, melatonin taken in the evening tends to shift the clock earlier (a phase advance, useful for eastward travel), while melatonin in the morning tends to shift it later (a phase delay). Because the light and melatonin curves are opposed, their timing rules point in opposite directions – a detail that matters when planning a shift, and a reason to follow a worked-out plan rather than guess.
On melatonin as a supplement or medicine: availability and legal status differ by country – for example, in the UK melatonin is generally a prescription medicine, while in the United States it is sold over the counter as a supplement with less consistent regulation of purity and dose. Evidence that appropriately timed melatonin can help with jet lag exists, but the right timing, whether it is suitable for you, and any dose are medical questions. jetlag.info deliberately gives timing concepts only and does not recommend doses. Talk to a doctor or pharmacist before use – especially if you are pregnant, travelling with children, take other medication, or have a medical condition.
Illustration (planned graphic)
A 24-hour timeline (inline SVG) with a shaded melatonin “band” that begins rising in the evening – labelled “DLMO, about 2-3 hours before sleep” – peaks in the middle of the night, and falls toward wake. A small sun/light-bulb icon over the daytime stretch, with a downward arrow on the band, shows that light suppresses melatonin. Optionally a faint mirror curve indicates that the melatonin phase response curve runs opposite to the light curve. Positions are meaningful, so this is rendered as data-accurate SVG rather than an illustration.
What it means for travel
Two takeaways. First, protect your natural melatonin rise on the night side of your trip by dimming lights and screens when your destination clock says evening, and get bright light when it says morning – light, working through the same clock, is the primary lever, and the light planner lays out those windows. Second, because supplemental melatonin’s effect depends entirely on timing and is a medical decision, treat any dose or product question as one for a pharmacist or doctor, not a calculator.
To see where your own biological night sits relative to your destination, use the body-clock tool.
Common questions
Does melatonin make you fall asleep?
It is primarily a timing signal, not a strong sedative. It tells the body it is night and can make sleep easier at the right biological time, but it does not force sleep the way a hypnotic drug does.
What is DLMO?
Dim light melatonin onset – the evening moment melatonin begins to rise under dim conditions, usually two to three hours before habitual sleep. Researchers use it as a precise marker of circadian phase.
Should I take a melatonin supplement for jet lag?
That is a medical decision. Its legal status and regulation vary by country, and the benefit depends heavily on correct timing. Discuss suitability, timing and dose with a doctor or pharmacist – especially during pregnancy, for children, or alongside other medication.
Sources
- Lewy AJ, Ahmed S, Jackson JML & Sack RL (1992). “Melatonin shifts human circadian rhythms according to a phase-response curve.” Chronobiology International, 9(5). Basis for the melatonin phase response curve.
- Lewy AJ and colleagues – work establishing dim light melatonin onset (DLMO) as a marker of circadian phase.
- NHS – “Melatonin” medicines information and “Jet lag” (nhs.uk); melatonin is generally a prescription medicine in the UK.
- U.S. Centers for Disease Control and Prevention – CDC Yellow Book, “Jet Lag” chapter, which discusses melatonin among possible aids and advises clinician input.
Directional timing statements (evening advances, morning delays) follow the peer-reviewed melatonin phase-response work; regulatory status is summarised from national health-service guidance and varies by country. No doses are stated.