Melatonin is an endogenous hormone signal primarily released at night, letting the body "know" it's biologically nighttime. As a supplement, melatonin is the same molecule but taken externally: an additional signal at a chosen time. In research, it is used for two purposes: either to support falling asleep or to shift the sleep rhythm when your internal clock doesn't align well with your desired bedtime (e.g., in cases of delayed sleep-wake phase disorder or jet lag).
What is melatonin?
In humans, melatonin is primarily produced and secreted by the pineal gland at night. Its production is controlled by a "central clock" in the brain, which is sensitive to light and dark. Consequently, (bright) light in the evening or at night can suppress melatonin release. In this sense, melatonin primarily serves as a night message: it helps synchronize bodily processes with the sleep-wake rhythm.
The difference between endogenous melatonin and melatonin as a supplement is not "what it is," but how it is administered. Endogenous melatonin follows a rhythm that adjusts to light, dark, and your internal timing. Orally taken melatonin is an extra, external signal. Depending on the dose, timing, and release form, this signal can primarily work around falling asleep, or it can be used to advance your sleep time in cases of a delayed rhythm.
Does melatonin work for sleep?
The honest answer is: sometimes, but the effect heavily depends on the situation. In adults with a "classic" form of insomnia, meta-analyses show small improvements on average, especially with prolonged-release melatonin. A recent meta-analysis reported an average of approximately 5–6 minutes shorter sleep onset latency and a small improvement in sleep efficiency with prolonged-release melatonin (Maruani et al., 2023).
At the same time, the evidence is not consistent. A meta-analysis on chronic insomnia found no clear improvement in sleep onset latency, total sleep time, or sleep efficiency in adults in the non-comorbid group (Choi et al., 2022). An RCT with 3 mg fast-release melatonin for 4 weeks in middle-aged individuals with primary insomnia also found no improvement in several sleep outcomes, although a decrease in "early awakenings" was reported (Xu et al., 2020).
Where the signal becomes stronger is when the problem is primarily a timing issue. In delayed sleep-wake phase disorder (DSWPD: a structurally delayed sleep rhythm), 0.5 mg fast-release melatonin, taken 1 hour before the desired bedtime, led to an average of 34 minutes earlier sleep onset than placebo (Sletten et al., 2018). In that study, melatonin was combined with a fixed bedtime, which shows that melatonin in research is often part of a rhythm-based approach and not just a "sleeping pill" (Sletten et al., 2018).
What are important characteristics of melatonin?
An important characteristic is that "melatonin" in studies does not mean a single, fixed protocol. The literature varies on three points that greatly influence the outcome.
Firstly, there is the release form. Fast-release is often used around the time of falling asleep or for phase shifting in a delayed rhythm, while prolonged-release is intended to be released longer throughout the night and thus more closely resemble a "night profile," especially in studies on insomnia (Maruani et al., 2023).
Secondly, the dose varies greatly. In the selected evidence, dosages range from 0.5 mg to 10 mg, depending on indication and study design.
Thirdly, timing is likely one of the biggest explanations for varying results. A dose-response meta-analysis with meta-regression suggests that earlier intake (in the order of hours before the desired bedtime) in RCT data is associated with greater improvement in sleep onset latency than a commonly used regimen such as 2 mg 30 minutes before bedtime (Cruz-Sanabria et al., 2024). This is not proof that "3 hours is always better," but it does underscore that timing is a core variable.
What is the difference between melatonin and sleep medication?
Melatonin acts via a different mechanism than classic sleep medications such as benzodiazepines and so-called Z-drugs. In simple terms: sleep medications usually directly interfere with systems that cause drowsiness and sedation, while melatonin is primarily a time signal that communicates "night." This difference is also reflected in research on sleep structure: in a small cross-over study in healthy adults, temazepam and zolpidem had clearer effects on sleep parameters than prolonged-release melatonin (Arbon et al., 2015).
In a large network meta-analysis of pharmacological treatments for insomnia, benzodiazepines and Z-drugs were on average more effective than melatonin in acute treatment, but they were also more frequently associated with side effects and withdrawal due to side effects (De Crescenzo et al., 2022). Within the same analysis, melatonin was not positioned as an intervention with clear overall benefits in the overall picture, which is consistent with the finding that melatonin has a small average effect in "classic" insomnia (De Crescenzo et al., 2022; Maruani et al., 2023).
When should you take melatonin?
The timing in studies is linked to the objective. For primary insomnia, an RCT used 3 mg fast-release melatonin 1 hour before bedtime, daily for 4 weeks (Xu et al., 2020). For DSWPD, 0.5 mg fast-release was taken 1 hour before the desired bedtime, at least five evenings per week, combined with a fixed bedtime (Sletten et al., 2018).
For shift work, a field RCT investigated 3 mg melatonin 30 minutes before the sleep period, with small improvements in sleep efficiency and sleep onset latency (Sadeghniiat-Haghighi et al., 2016). For jet lag, regimens varied, but intake was always aimed at sleeping at the destination (Tortorolo et al., 2015).
A nuance from the dose-response literature is that "earlier intake" in RCT data is statistically associated with greater gains in sleep onset latency, but this is model-based and does not establish a universal regimen (Cruz-Sanabria et al., 2024).
Is melatonin bad for you?
Based on the selected RCTs and meta-analyses, melatonin appears to be generally well-tolerated, with mostly mild side effects such as drowsiness, headache, or dizziness. In short-term adult RCTs, no serious side effects were reported (Xu et al., 2020; Sletten et al., 2018). In a safety meta-analysis of higher doses (≥10 mg), no increase in serious side effects or withdrawal due to side effects was found in a low-risk subset for bias, but an increase in non-serious side effects was observed (RR 1.40). At the same time, adverse event reporting was limited in many studies, undermining long-term certainty (Schrire et al., 2022).
It is also important to note what is NOT in the provided evidence: within this selection, there is no robust, target group-specific evaluation for pregnancy/breastfeeding, and long-term use over months to years has simply not been investigated in many RCTs.
Conclusion: what is melatonin?
Melatonin is an endogenous nocturnal signal that helps organize your internal timing. As a supplement, it can be useful in some situations, but the effect is not equally significant everywhere. In adults with insomnia, the average effects in meta-analyses are small, and the evidence is mixed, whereas for delayed sleep-wake phase disorder (DSWPD), a clearer effect has been found when melatonin is combined with fixed bedtimes. The practical essence from the literature is that dose, release form, and especially timing make a difference, and many questions about long-term use have not yet been solidly answered in RCTs.
References
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