Melatonin Side Effects: What the Research Actually Shows

The side effects trials actually record

The most complete look at melatonin's side effects pooled 37 randomized controlled trials using 0.15 to 12 mg a day, most of them lasting four weeks or less (Besag et al., 2019). The most common side effects were daytime sleepiness (1.66% of participants), headache (0.74%), dizziness (0.74%) and a drop in body temperature (0.62%). Most went away within a few days or as soon as people stopped taking it, and the authors describe melatonin as generally safe and well tolerated.

Two cautions about that reassuring picture. Only 8 of the 37 trials were rated good quality, and almost none ran for more than a few months. NIH's National Center for Complementary and Integrative Health says information on the long-term safety of melatonin supplements is lacking.

Does melatonin cause next-day grogginess?

Melatonin leaves the body quickly: its half-life is about 45 minutes (Harpsøe et al., 2015). In controlled studies of low evening doses, 0.3 mg and 1 mg shortened the time it took to fall asleep and caused no hangover on next-morning mood and performance tests (Zhdanova et al., 1995). A 2 mg prolonged-release dose taken at bedtime by adults averaging 59 years old did not impair memory, reaction time or simulated driving the next morning (Otmani et al., 2008).

Timing matters, though. When volunteers took melatonin at midday, even doses from 0.1 to 10 mg made them sleepier and worse at a vigilance task while it was active (Dollins et al., 1994). And the studies above used small doses; many products on US shelves contain 5 or 10 mg per serving, and there is less research at those amounts. If you wake up groggy, the dose, how late you took it, and how much the product really contains are all worth checking.

Vivid dreams and nightmares

Stronger dreams are one of the most talked-about melatonin complaints online, but the published evidence is thin. In the 37-trial review, nightmares appear only on a short list of rarer events, with no rate given. We found no solid study showing that ordinary melatonin supplements cause vivid dreams, so we will not claim they do.

The dose problem

Research shows how little melatonin can be needed. In a laboratory study, 0.3 mg taken in the evening raised blood melatonin to within the normal night-time range and shortened the time to fall asleep (Zhdanova et al., 1996). Store products are often much stronger: the melatonin gummies in a 2023 study were labeled at 1 to 10 mg per serving (Cohen et al., 2023).

What is in the bottle can also differ from the label. In that 2023 analysis of 25 US gummy products, actual melatonin ranged from 74% to 347% of the labeled amount, only 3 were within 10% of the label, and one contained no melatonin at all, only CBD (Cohen et al., 2023). An earlier analysis of 31 supplements found melatonin from 83% below to 478% above the label, and 8 also contained serotonin (Erland and Saxena, 2017).

Use has grown fast. The share of US adults taking melatonin rose from 0.4% in 1999–2000 to 2.1% in 2017–2018, and it was highest, 2.9%, among people aged 45 to 64 (Li et al., 2022).

Tolerance, dependence and stopping

There are no dedicated studies of melatonin tolerance or withdrawal. The best available evidence is indirect: a review of prescription prolonged-release melatonin trials in adults aged 55 and over, written by researchers linked to that product's developer, reported no rebound, withdrawal or hangover effects at 2 mg a day for three months (Lemoine and Zisapel, 2012). In the 37-trial review, side effects went away when people stopped.

If melatonin is not working for you

Melatonin is a timing signal: it tells your body clock it is night. It is best supported for timing problems such as jet lag, and it is not designed to settle a stressed, switched-on mind. If that is what keeps you awake, a different approach may fit better: see the best melatonin alternatives, α-casozepine vs melatonin and how to fall asleep fast.