Melatonin May Have Harmful Effects After All: Type 2 Diabetes

New research reveals that melatonin may have to harmful effects, potentially even leading to type 2 diabetes.


Melatonin, a hormone that is normally produced naturally by the body, helps us to regulate our circadian rhythm—our sleep/wake cycle. However, melatonin is often taken as a supplement for people who have difficulty sleeping for various reasons. It’s one of the first things many therapists suggest when someone complains of insomnia.

There is a lack of compelling evidence that melatonin supplements can help with insomnia. Meta-analyses show that people taking melatonin fall asleep about four minutes faster, on average, than people taking a placebo drug, and may sleep about 10-15 minutes longer. But at the very least, it has been considered safe—so even if it doesn’t work, what’s the harm?

Now, a new article reveals that melatonin may have harmful effects after all—potentially leading to type 2 diabetes.

The lead authors were Marta Garaulet and Jingyi Qian from the Medical Chronobiology Program at Brigham and Women’s Hospital, Boston. The paper, published in Trends in Endocrinology and Metabolism, also included researchers from Harvard, MGH, and the Broad Institute (affiliated with MIT and Harvard), a medical research organization that focuses on genomics and large datasets.

Garaulet and Qian’s article presents a theory that serves to unify what was previously considered contradictory data. In previous studies, melatonin supplementation was sometimes—inconsistently—shown to impair glucose tolerance. Impaired glucose tolerance leads to type 2 diabetes.

The article synthesizes results from various studies. Their findings indicate that when taken during the day—or when taken at night by people who eat late in the evening—melatonin can impair glucose tolerance, leading to type 2 diabetes.

Eating late in the evening—when the body expects to be fasting and sleeping—is already associated with an increased likelihood of obesity and diabetes. This is because—as a Psychiatric Times article explains: “An evening meal will result in higher glucose and insulin levels compared with the identical meal eaten early in the day.”

Taking melatonin supplements appears to exacerbate this effect. According to Garaulet and Qian, increasing melatonin in the daytime also results in higher glucose and insulin levels, as does eating at around the same time one is taking melatonin.

Optimally, low melatonin during the day and when eating is associated with increased glucose tolerance. Likewise, high melatonin at night during fasting and sleeping is related to pancreatic β cell recovery. Both of these situations help prevent type 2 diabetes.

However, Garaulet and Qian report that there is also a genetic element at play. The MTNR1B gene influences melatonin receptors, and people who have mutations (polymorphisms) in this gene may experience higher fasting blood glucose levels and thus increased risk for type 2 diabetes.

According to Garaulet and Qian, people with polymorphisms in the MTNR1B gene may be particularly adversely affected by the timing of meals: “We have shown that consuming a late versus an early dinner, which was associated respectively with high and low melatonin concentrations, impairs glucose tolerance in MTNR1B risk allele carriers but not in noncarriers.”

People with polymorphisms in the MTNR1B gene may also have higher melatonin concentrations for a longer period of time—which means that having breakfast in the morning may coincide with still-high melatonin levels.

Similarly, standard doses of melatonin—such as 5 mg oral fast release—can also result in still-high melatonin levels in the morning. According to the authors, such a dose causes “plasma melatonin levels to remain 10-fold higher than physiological peak levels even 6 [hours] after administration, which could result in elevated melatonin levels the following morning.”

The authors suggest that changing eating habits may minimize the harmful effects of melatonin. This is true for people taking supplements—but also true for the general population, especially those with MTNR1B polymorphisms.

  • Have dinner earlier, and don’t snack in the evening.
  • Have breakfast later, rather than just after waking.
  • If taking supplements, take them later in the evening (at least 2 hours after eating).



Garaulet M, Qian J, Florez JC, Arendt J, Saxena R, & Scheer FAJL. (2020). Melatonin effects on glucose metabolism: time to unlock the controversy. Trends in Endocrinology and Metabolism, 31(3). (Link)

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Peter Simons
Peter Simons was an academic researcher in psychology. Now, as a science writer, he tries to provide the layperson with a view into the sometimes inscrutable world of psychiatric research. As an editor for blogs and personal stories at Mad in America, he prizes the accounts of those with lived experience of the psychiatric system and shares alternatives to the biomedical model.


  1. And here:

    I would attend this conference at the new tower of the World Trade Center, in the building where the New York Academy of Science is headquartered to hear about circadian rhythm, light and cancer. I would work up a poster that drew from Technology Assessment Office (see: and then ponder the meaning of what a city did to expand for UPS, to hire college students to work their conveyor lines at night) that flies in and out and night. All the while, the larger health insurance company stages fitness runs, sponsors events, when the experience from the mind side of the body, has not been so favorable where the horse rules along with a serious history of doping. And then today, to try and drive home, to get back to my apartment in a city that has installed concrete barriers and dump trucks to shut off access to downtown—- Are we opened for business ? These actions seem to be a strange way to ramp up to making America Great Again? And by Saturday, to realize a new Judge? On the bench? George’s eyes are watching!

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    • A follow-up question: Does one think a city is an organism, that should work 24/7? From the meeting at the NYAS, melatonin intake would be increased along with sleep in a blackened room and improved diet to reduce the cancers (up to 50% in women) as reported by Dr. Stephen Bloch of the Bloch Cancer Institute. There were also some researchers from Texas and S.C. that also affirmed these positions. So, this news helps to understand a way forward that will be addressed. Tks.

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      • Sorry, I confused the spelling of Bloch and Block. The Block that presented at the conference has a cancer clinic in Chicago, the other Bloch would create the Bloch Cancer Foundation and Gardens, and owned the H&R Block Tax Company. However, if the prevailing social structure can not understand how to advance credit by which We, can realize the importance of our LIFE as opposed to death, then who or what is avoiding a shift in the nature of justice?

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  2. Very unlikely these ominous correlations regarding melatonin supplementation have any clinical validity. In supplement form, melatonin has a half-life of only a few hours.

    As little as 0.33mg melatonin has found to be effective to trigger sleep, especially if taken at night and surroundings are dark. That’s basically what it’s good for, because of the half-life, the effect wears off a few hours later, with natural circadian rhythm ending endogenous melatonin production and ramping up cortisol as dawn approaches. Melatonin is not a sleeping pill.

    If use of the supplement is keep to minimally effective doses, adverse effects are probably close to zero.

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    • Half life of various drugs
      Cocaine 1 hour
      Heroin 30 minutes

      Half life means the time it typically takes for half the original substance to be metabolized and/or eliminated from someone’s system. It is not a reflection on the harms or benefits of the substance. It isn’t even an actual reflection on the duration of the substances actions. Many drug effects last last long after the original substance has been metabolized. These effects are sometimes called “withdrawal” “hangovers” “addiction”

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      • Half-life has to do with immediate drug effects, including withdrawal. It doesn’t account for damage and changes in the brain or body resulting from the drug use. Especially after long-term use, these changes can be profound and take a LONG time to repair themselves, if it ever even happens.

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        • I wonder if someone had been on a tricyclic or another older chemical imbalance causing drug how would that impact their perceived harm and benefit from a newer SSRI? It’s another confounding variable. We can’t tell if the new drug is actually helping the “illness” or symptoms from the past drug induced illness psychiatry created.

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