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Melatonin is a hormone secreted nightly by the pineal gland that signals darkness to the body's circadian system, coordinating the timing of sleep onset, core body temperature reduction, and other overnight biological processes. It functions as a chronobiotic, resetting the internal clock rather than inducing sedation, and its rhythm is suppressed by exposure to blue-spectrum light.
As a chronobiotic, melatonin shifts the timing of the circadian clock; it is not a sedative in the pharmacological sense.
How it works
Melatonin is synthesised from tryptophan via serotonin in the pineal gland. The synthesis pathway is entirely gated by light: photoreceptive cells in the retina called intrinsically photosensitive retinal ganglion cells (ipRGCs) detect ambient light and transmit a suppressive signal via the superior cervical ganglion, halting melatonin production within minutes of light exposure 2. Secretion follows a precise nocturnal window, with concentrations beginning to rise approximately two hours before habitual sleep time (the dim-light melatonin onset, or DLMO), peaking at 0.1-0.9 ng/mL in the middle of the night, then falling to near zero by morning 2 1.
Once secreted, melatonin binds to MT1 and MT2 receptors in the suprachiasmatic nucleus (SCN), the brain's primary circadian pacemaker 1. This binding does not generate sleep directly; rather, it consolidates the circadian signal and lowers arousal threshold, creating the physiological conditions under which sleep can begin. Think of the SCN as a master clock, and melatonin as the daily time-stamp it needs to synchronise its outputs across every organ system.
Blue-spectrum light, peaking in sensitivity around 480 nm, is the primary environmental disruptor of this system. Evening exposure from screens or overhead lighting activates the same ipRGC pathway that suppresses nocturnal production, and can delay DLMO by 1.5 to 3 hours 2. A person exposing themselves to bright screens until midnight may not experience natural melatonin onset until 2 or 3 a.m., regardless of intended sleep time.
Example
A professional athlete trains until 8 p.m. and then spends two hours reviewing video footage on a bright laptop screen. Despite feeling tired, they struggle to fall asleep before 1 a.m. and wake groggy at 7 a.m. The culprit is not insufficient sleep drive but a light-suppressed melatonin signal: their DLMO has been pushed past midnight, so their circadian window for sleep opened hours after they got into bed.
Dimming screens after 8 p.m. is not aesthetic preference; it is circadian intervention.
Why it matters
Because melatonin acts on timing rather than sleep generation, supplementing at the wrong phase of the circadian cycle can delay the clock further rather than advance it 1 2. A dose-response meta-analysis of 26 randomised controlled trials found that exogenous melatonin reduces sleep onset latency and increases total sleep time most effectively at approximately 4 mg taken three hours before target bedtime 3. That timing and dosage differ substantially from the single 10 mg tablet taken immediately before bed that most users default to.
Compounding the timing issue is a product reliability problem. A 2023 survey of over-the-counter gummy products found 88% were inaccurately labelled, with actual melatonin content ranging from 74% to 347% of the declared dose 4. Endogenous peak melatonin is only 0.1-0.9 ng/mL nightly, meaning even a 0.5 mg supplement exceeds physiological concentrations. Standard 5-10 mg tablets deliver five to a hundred times the endogenous peak, offering no additional chronobiotic benefit while increasing next-day sedation risk 3 2.
What is the correct melatonin dosage for sleep?+
A dose-response meta-analysis of 26 randomised controlled trials identified approximately 4 mg taken three hours before target bedtime as the most effective protocol for reducing sleep onset latency and increasing total sleep time {{cite:10.1111/jpi.12985}}. This is lower than most OTC tablets and requires earlier administration than most users practise.
Does melatonin make you sleepy, or does it just time your sleep?+
Melatonin is a chronobiotic, not a sedative. It binds to receptors in the suprachiasmatic nucleus to consolidate circadian timing and lower arousal threshold, creating a permissive window for sleep rather than compelling it {{cite:10.1016/j.smrv.2004.05.002}}. This is why taking it at the wrong biological time may have little or no effect.
How does evening light exposure affect melatonin?+
Evening light, particularly blue-spectrum wavelengths peaking around 480 nm, activates intrinsically photosensitive retinal ganglion cells (ipRGCs) that suppress pineal melatonin production {{cite:10.1111/bph.14116}}. Consistent screen use until late evening can delay melatonin onset (DLMO) by one and a half to three hours, effectively pushing the natural sleep window back by the same amount.
Are OTC melatonin supplements reliable?+
Often not. A 2023 JAMA survey found that 88% of over-the-counter melatonin gummies were inaccurately labelled, with actual doses ranging from 74% to 347% of stated content {{cite:10.1001/jama.2023.2296}}. Even accurate products typically contain 5-10 mg doses that far exceed the endogenous physiological peak of 0.1-0.9 ng/mL, offering no additional chronobiotic benefit {{cite:10.1111/jpi.12985}}.
