What melatonin actually does

Melatonin is a hormone secreted by the pineal gland in response to darkness, and its core job is to communicate the time of night to the rest of the body, not to sedate it. Brzezinski 1997 (N Engl J Med) describes it as the chemical expression of darkness: levels are near-undetectable during the day, rise in the evening, peak in the middle of the biological night, and fall before waking. The brain reads this rising signal as night and uses it to time sleep, lower core body temperature, and coordinate peripheral clocks. This is why melatonin is best understood as a timing cue. It tells the system when it is night rather than forcing the brain into sleep the way a benzodiazepine or a Z-drug does.

Melatonin production is controlled by the master clock. The suprachiasmatic nucleus (SCN) drives a multi-step pathway that, in darkness, switches on the pineal enzyme that converts serotonin to melatonin. Light is the override: signals from melanopsin-containing retinal cells reach the SCN and shut the pathway down, which is why even ordinary indoor light at night suppresses melatonin. Lewy et al. 1980 (Science) first showed that bright light at night sharply lowers human nocturnal melatonin, establishing that the hormone is gated by light, not just by clock time. So the body makes its own melatonin every night on schedule, and evening light exposure is the main thing that blunts it.

As melatonin rises in the evening, it contributes to the physiological lead-up to sleep: a fall in core body temperature, a drop in alertness, and the opening of the circadian sleep gate. It acts on MT1 and MT2 receptors in the SCN itself, feeding back onto the clock and reinforcing the night signal. Its effect on sleep propensity is real but modest and indirect, working through these timing and temperature pathways rather than through the direct sedation produced by hypnotic drugs. The clearest marker of all this is dim light melatonin onset (DLMO), the evening rise measured under dim conditions, which is the gold-standard phase marker chronobiology labs use to read where someone's clock actually sits.

Taking melatonin as a supplement adds an artificial pulse of the same signal. Its strongest, most reliable use is shifting circadian timing: low-dose melatonin can advance or delay the clock depending on when it is taken, mapped by the melatonin phase response curve in Burgess et al. 2010 (J Physiol). Taken in the late afternoon or early evening it advances the clock earlier; taken late at night it can delay it. Its direct effect on falling asleep is smaller than most people expect: the meta-analysis by Brzezinski et al. 2005 (Sleep Med Rev) found only a modest reduction in sleep-onset latency. That is the consequence of its mechanism. It is a timing cue, so it is most useful for a mistimed clock (jet lag, delayed sleep phase, shift work) and least useful as a nightly knockout pill for an otherwise well-timed sleeper.

Because melatonin is a signal rather than a sedative, flooding the system with more of it does not add proportional benefit. Zhdanova et al. 2001 (J Clin Endocrinol Metab) found that 0.3 mg restored sleep as well as 3 mg in older adults, and only the lower dose kept blood melatonin within the normal physiological range. Retail doses of 3-10 mg push levels to 10-100x the natural nocturnal peak and keep them elevated into the morning, which is what produces next-day grogginess without improving the sleep itself. For how much to take and when for a given goal, see the melatonin dosing guide; the short version is that timing is the lever and the physiological dose is 0.3-1 mg.

Before reaching for a supplement, the highest-yield move is to stop suppressing the melatonin you already make. Bright and short-wavelength (blue) light in the evening is the dominant suppressor (Lewy et al. 1980), so dimming lights and reducing screen brightness in the 2-3 hours before bed protects the natural evening rise; this is the rationale behind evening blue-blocking glasses for people exposed to bright screens at night. Conversely, getting bright light in the morning sharpens the day-night contrast that makes the evening rise cleaner. This article is educational and not medical advice; anyone pregnant, on anticoagulants or immunosuppressants, or considering daily use should discuss melatonin with a clinician.