Melatonin: role in circadian rhythms, biological action, uses, and safety

Production and timing

In mammals most circulating melatonin is synthesized in the pineal gland, a small neuroendocrine structure that lies outside the blood–brain barrier and releases hormone into the bloodstream at night. Production is driven by a neural pathway from the retina so that light at night suppresses synthesis and darkness promotes it. Melatonin timing and amplitude are commonly used as markers of circadian phase in research and clinical practice, for example the dim-light melatonin onset (DLMO).

Extra-pineal sites and metabolism

Melatonin is also found in other tissues, including the retina and the gastrointestinal tract, and is present in many plants and microbes. These extra-pineal pools have local functions in some species, though circulating night-time melatonin in mammals mainly reflects pineal secretion. The hormone is metabolized in the liver and excreted in urine; metabolic pathways and clearance can vary with age and with drugs that affect liver enzymes (regulatory notes).

Mechanisms of action

Melatonin signals through specific receptors (commonly called MT1 and MT2) located in the brain and peripheral tissues, influencing sleep propensity, thermoregulation and the timing of circadian oscillators. In addition to receptor-mediated effects, melatonin has direct antioxidant activity and can scavenge free radicals, with experimental reports of protective actions on cellular components including DNA and mitochondria (antioxidant research, cell protection summaries).

Clinical uses and evidence

Melatonin is widely used to address sleep-related problems and to adjust circadian timing. Common applications include short-term management of jet lag, assistance in shift work–related circadian adaptation, adjunctive treatment for some forms of insomnia and clinical use in selected pediatric circadian disorders under supervision. Randomized trials and meta-analyses indicate modest benefits for some sleep outcomes; effectiveness depends on timing, dose and formulation (immediate versus prolonged release). For guidance and regulatory context see authoritative reviews and product information (product distinctions).

Availability and regulation

Regulatory status differs by jurisdiction. In the United States melatonin is commonly sold as a dietary supplement and regulated accordingly, not as a prescription drug (FDA classification, supplement rules). Some countries have prescription-only, timed-release melatonin products approved for defined populations, such as older adults with sleep disorders; these approvals are based on regulatory review of safety and modest efficacy.

Safety, interactions and practical considerations

Melatonin is generally well tolerated, but transient side effects such as daytime sleepiness, headache and dizziness are reported. Because it affects sleepiness and may interact with other central nervous system depressants, caution is advised when combining melatonin with sedatives or alcohol. Potential drug interactions include effects with anticoagulants, immunosuppressants and medicines metabolized by hepatic enzymes; clinical review is recommended before use. Special populations such as pregnant or breastfeeding people, children and those with chronic medical conditions should seek medical advice before using melatonin.

Practical advice

• Timing of administration relative to a person’s circadian phase is a major determinant of effect: appropriately timed small doses can shift circadian timing, while mistimed use may cause unwanted phase shifts.
• Formulation matters: immediate-release preparations may promote sleep onset, whereas extended-release formulations aim to sustain concentrations overnight.
• Product quality varies among over-the-counter supplements; when treatment is clinically indicated, regulated medicinal products provide defined dosing and release characteristics.

Distinctions and research directions: endogenous melatonin differs from synthetic melatonin receptor agonists and from regulated prescription formulations designed for specific clinical uses. Ongoing research continues to refine optimal timing, dose-response relationships and long-term safety, and to clarify roles in ageing, metabolic health, and protection against oxidative stress. For further information consult clinical guidelines and systematic reviews (biology reviews, clinical guidance, regulatory documents, product information, antioxidant studies, cell protection sources, FDA notes, supplement rules).

Note: This article summarizes broadly established aspects of melatonin biology and clinical use. It does not replace individualized medical advice.