Melatonin blunted insulin release in carriers of a common diabetes gene variant

Melatonin is sold over the counter as a sleep aid, and its use has climbed steeply — the authors of a new trial note it has quintupled in recent decades [s1]. It is also a hormone, not merely a supplement, and one of the places it acts is the insulin-producing beta cell of the pancreas. A small, carefully controlled study in Diabetes Care set out to characterise what that does to blood-sugar control, and whether a common gene variant changes the picture [s1].

The variant matters. A frequent version of the melatonin receptor 1B gene (MTNR1B) is one of the better-established common genetic risk factors for type 2 diabetes, and it changes how the beta cell responds to melatonin. That raises an obvious question for a widely used product: if you carry the risk variant, does taking melatonin at night nudge your metabolism in the wrong direction?

What the trial did

The study was a randomised, double-blind, placebo-controlled crossover trial in 21 healthy participants of European ancestry — 10 carriers of the MTNR1B risk allele and 11 non-carriers [s1]. Each person completed a tightly controlled five-day laboratory protocol and received 5 mg of oral melatonin or placebo, in randomised order, on two non-consecutive days [s1]. Glucose dynamics were measured with insulin-modified intravenous glucose-tolerance tests and analysed with a mathematical minimal model that separates how much insulin the beta cells release from how sensitive the body's tissues are to it [s1]. The trial was run at Brigham and Women's Hospital and registered as NCT02642640 [s2].

A crossover design is the right tool here: each participant serves as their own control, which matters when the whole question is whether one gene group responds differently from another.

What it found

The effects landed almost entirely on the carriers. Compared with placebo, melatonin worsened glucose tolerance and reduced early C-peptide responses in the risk-allele carriers — by 11.7% (95% CI 1.0 to 22.3) and −19.2% (−33.9 to −1.2) respectively (adjusted P<0.05) — but not in the non-carriers [s1].

The mechanism underneath those numbers is the striking part. In carriers, the impairment came from a 40% suppression of glucose-stimulated first-phase beta-cell responsivity (95% CI −52.4 to −24.3; adjusted P=0.0003) — the rapid burst of insulin the pancreas releases in the first minutes after glucose appears — together with a slower insulin-induced decline in the second-phase insulin secretion rate (64.3%, 23.1 to 119.2; adjusted P=0.001), a sign that the normal feedback braking of insulin release was dysregulated [s1]. First-phase insulin release is one of the earliest things to fail on the road to type 2 diabetes, so a supplement that blunts it in genetically susceptible people is not a trivial finding.

The other side of the same coin

The trial also caught a consequence that cuts the other way. In carriers, melatonin prevented insulin-induced hypoglycaemia: there were zero such events on melatonin against seven on placebo (P=0.001), and again no effect in non-carriers [s1]. The same blunting of insulin dynamics that worsens glucose tolerance also dampens the swing into low blood sugar. That is not a benefit anyone should chase with a sleep supplement, but it underlines that the drug is genuinely acting on insulin biology, not producing a statistical fluke.

How much to make of it

The limits are real and should be stated first. This is 21 healthy young adults, not people with diabetes, studied for two dosing days under laboratory conditions with an intravenous glucose challenge rather than ordinary meals [s1]. It measures physiology, not clinical outcomes: nobody developed diabetes in a two-day study, and it cannot say how a nightly dose over months or years would play out. The carrier subgroup was ten people.

What gives the result its weight is the mechanistic coherence and the funding. The effects track a known gene–drug interaction, they point in a consistent direction across several independent measures, and the work was supported by the US National Institutes of Health, Harvard University and the American Diabetes Association rather than by a supplement maker [s1]. Independent, publicly funded physiology is exactly the kind of evidence least likely to be shaped by a commercial interest in the answer.

The practical reading is measured. For most people melatonin's metabolic effect here was negligible. For the sizeable minority who carry the MTNR1B risk variant — and who would not know it without genotyping — a nightly hormone that suppresses the first-phase insulin response deserves more scrutiny than an over-the-counter sleep aid usually gets, and a larger, longer trial to settle whether the laboratory signal matters in daily life.

This article is informational and does not constitute medical advice.

Sources

• [s1] Melatonin Impairs Glucose Tolerance, First-Phase Insulin Secretion, and Insulin Feedback Inhibition; Interaction With MTNR1B Diabetes Risk Variant. Diabetes Care, 25 June 2026. https://doi.org/10.2337/dc26-0164
• [s2] Interaction of Melatonin and MTNR1B Genotype on Glucose Control - Study 1. ClinicalTrials.gov identifier NCT02642640, US National Library of Medicine. https://clinicaltrials.gov/study/NCT02642640