Can Melatonin Cause Bradycardia or a Slow Heart Rate?

Melatonin can cause bradycardia, but it appears to be rare. At least one published case report documents a healthy man whose heart rate dropped to 39 beats per minute after taking melatonin, while controlled studies in healthy volunteers generally show much more modest heart-rate changes on the order of a few beats per minute, or no change at all. The disconnect between the dramatic case and the calmer trial data points to a story that depends on individual physiology, dose, and context.

The Case That Put Bradycardia on the Radar

The most cited report involves an otherwise healthy man who arrived at an emergency department with a heart rate of 38 and was found on ECG to have sinus bradycardia at 39 beats per minute. The only new variable was melatonin. After about four hours of monitoring, his heart rate climbed back into the 50s on its own, and he remained symptom-free throughout the observation period before being discharged without any intervention.¹ That spontaneous recovery is important: the bradycardia resolved once melatonin’s effects wore off, suggesting a direct, reversible pharmacological cause rather than any underlying heart disease.

Case reports sit at the bottom of the evidence hierarchy because they describe a single patient’s experience, and you cannot generalize from one person. But they serve a different purpose here: they flag a biologically plausible adverse effect that larger trials may not have been powered to detect, especially when the event is uncommon. And the mechanism behind this particular case fits neatly with what researchers already know about melatonin’s cardiovascular effects.

How Melatonin Shifts the Balance Between “Rest” and “Fight” Signals

Your autonomic nervous system has two branches that push your heart rate in opposite directions. The sympathetic branch speeds things up, and the parasympathetic branch, acting through the vagus nerve, slows things down. Melatonin tips this balance toward the slow-down side. Research has linked melatonin to enhanced vagal tone and parasympathetic function, along with reduced sympathetic tone and lower circulating levels of norepinephrine, the chemical that tells your heart to beat faster.¹

A study in healthy men confirmed this autonomic shift quantitatively. Within an hour of taking melatonin, the interval between heartbeats lengthened compared to placebo, markers of parasympathetic activity rose, the ratio of sympathetic to parasympathetic power fell, and blood pressure dropped.² All of those changes point in the same direction: the heart quieting down. In most people, this shift is subtle enough that you would never notice. But if someone already has strong vagal tone, or if the dose is high enough, that quieting can overshoot into clinically slow territory.

Melatonin receptors themselves also play a role in the cardiovascular system. Two receptor subtypes, MT1 and MT2, are distributed in blood vessels and help regulate whether arteries constrict or dilate. At low concentrations, melatonin tends to promote constriction through MT1; at higher concentrations, it favors dilation through MT2.³ This concentration-dependent behavior helps explain why dose matters so much for cardiovascular outcomes, and why the effects of a low-dose supplement and a high-dose supplement could look quite different.

What Happens to Heart Rate in Controlled Studies

The case report is dramatic, but the broader trial data paints a more nuanced picture. When researchers gave 3 mg of melatonin to five healthy men and tracked their vitals, blood pressure dropped significantly, yet heart rate did not change in a meaningful way.⁴ Another study found that daytime melatonin lowered heart rate by roughly 3.5 beats per minute, a small but statistically detectable decline.⁵ That second study also found that a marker of sympathetic activity to the heart decreased, but a marker of parasympathetic activity stayed unchanged, suggesting the heart-rate dip came from dialing down the “speed up” signal rather than cranking up the “slow down” one.

These results matter for everyday supplement users. A drop of three or four beats per minute is within normal physiological variation and is not something you would feel. It would not show up as “bradycardia” on any clinical definition, which typically starts below 60 beats per minute for adults. For the vast majority of healthy people taking standard doses, melatonin nudges heart rate down slightly without crossing into worrisome territory.

Animal studies broadly agree with the human data while adding one useful wrinkle. In a comparison between melatonin, zolpidem, and diazepam in rats, melatonin did not produce the tachycardia and hypertension seen with diazepam, confirming its relatively benign cardiovascular profile at typical doses.⁶ Among common sleep aids, melatonin’s heart-rate footprint appears to be one of the smallest.

Why Dose Is the Critical Variable

The animal literature suggests that melatonin’s heart-rate effects are dose-dependent. Rat studies have shown that higher doses produce more pronounced reductions in heart rate, mean arterial pressure, and serotonin release in the brain, with the dose-response curve becoming steeper at the upper end.¹ You cannot directly translate rat pharmacology into human dose equivalents, but the principle is intuitive: more hormone means a bigger effect on the pathways it acts on.

Human supplementation complicates this because the doses people actually take vary enormously. Melatonin supplements are available over the counter in many countries at doses ranging from fractions of a milligram up to 10 or even 20 mg. The physiological dose your pineal gland produces at night is on the order of tenths of a milligram. A 10-mg tablet delivers many times more than your body would ever produce on its own, which pushes blood levels into a range where cardiovascular effects become more likely.

A study of a melatonin-related compound tested at escalating doses in humans found no consistent changes in vital signs as a function of dose or time, and specifically noted no association with hypothermia, bradycardia, or hypotension.⁷ That finding offers some reassurance, though it involved a synthetic analog rather than melatonin itself, and the doses studied were within a controlled pharmaceutical range. It is worth noting that the severe bradycardia case report does not specify the patient’s exact dose, which limits how much we can say about where the threshold sits for a dangerous drop in heart rate.

The Problem with What Is Actually in the Bottle

Even if you carefully choose a low dose, there is no guarantee the tablet contains what the label says. An analysis of commercially available melatonin products found that actual melatonin content ranged from 83% less than the label claimed to 478% more. Lot-to-lot variability within the same product was as high as 465%. The researchers also found serotonin, a controlled substance, in eight of the supplements they tested.⁸

This variability has direct implications for heart-rate effects. If melatonin’s cardiovascular impact is dose-dependent, and a pill labeled “3 mg” might actually contain anywhere from half a milligram to nearly 15 mg, then some people could be unknowingly taking doses high enough to push their heart rate lower than expected. It also means that someone who takes the same product from a different lot could have a different physiological experience each time. For anyone concerned about cardiac effects, choosing a product that has been independently tested for content accuracy is one of the few practical steps available.

When Melatonin Meets an Already-Slow Heart

Bradycardia from melatonin is most concerning for people who already have a tendency toward a slow resting heart rate. Athletes, older adults with age-related conduction system changes, and anyone on medications that slow the heart, such as beta-blockers or certain calcium channel blockers, start from a lower baseline. Adding melatonin’s parasympathetic boost on top of an already slow rate could push someone below a safe threshold.

The dynamics in people with existing heart disease are not straightforward. In a study of patients with coronary artery disease who were classified as “nondippers,” meaning their blood pressure did not drop enough during sleep, 90 days of evening melatonin actually increased mean 24-hour heart rate overall. The response was not uniform, though: some patients showed more of a blood-pressure effect, while others showed more of a heart-rate acceleration.⁹ That finding might seem paradoxical given everything discussed above, but it likely reflects the difference between acute effects in healthy people and chronic effects in a population with disturbed autonomic regulation. A review of the literature has proposed that melatonin’s chronobiological action may help normalize an insufficient nocturnal decline in heart rate, essentially restoring a healthier day-night rhythm rather than simply pushing the rate down.¹⁰

This distinction between acute heart-rate lowering and long-term rhythm normalization is one that gets lost in simple safety discussions. For someone with coronary artery disease and an already-elevated resting rate, melatonin’s effects might actually be beneficial over weeks and months. For a healthy person with low resting heart rate who takes a large dose before bed, the same pharmacology could produce the opposite practical outcome.

Children, Neonates, and Overdose Scenarios

Melatonin use in children has grown rapidly, which raises the question of whether younger populations are at particular risk for cardiac effects. The limited evidence is somewhat reassuring on the bradycardia front specifically. In a study of neonates receiving melatonin infusions during therapeutic hypothermia, a setting where vital signs were being monitored continuously, researchers found no clinically significant changes in vital signs during or after administration.¹¹

In a published case of melatonin overdose involving extreme drowsiness and a very low responsiveness score, the patient’s pulse was recorded at 86 beats per minute, which is entirely normal.¹² The major clinical concern in that case was the altered consciousness, not cardiac rhythm. While one overdose case cannot rule out heart-rate problems at extreme doses, it does suggest that profound sedation seems to be the more consistent danger of taking too much melatonin, at least on the basis of published reports.

The Link Between Low Natural Melatonin and Heart Problems

An interesting flip side to the “does melatonin slow the heart” question is what happens when people have too little melatonin. Patients with type 2 diabetes and autonomic neuropathy, a condition where the nerves controlling heart rate and blood pressure are damaged, have been found to have significantly lower melatonin levels both at night and during the day compared to healthy controls. Their nocturnal melatonin levels correlated positively with heart rate variability measures, meaning that lower melatonin was associated with a less flexible, less responsive heart-rate pattern.¹³

This correlation does not prove that low melatonin causes autonomic damage, and it could easily be the other way around: damaged autonomic nerves might impair pineal gland function. But it highlights that melatonin is embedded in the body’s cardiovascular regulation machinery in ways that go well beyond simple sedation. Your body uses melatonin as one of the signals that coordinates the nighttime dip in heart rate and blood pressure, and disruption of that signal, whether from disease or from flooding the system with a large exogenous dose, can produce cardiac effects in either direction.

Melatonin’s Effect on the Heart’s Internal Clock

Heart rate follows a circadian rhythm that peaks in the afternoon and reaches its lowest point in the early morning hours. Heart rate variability follows its own parallel cycle. Research has shown that exogenous melatonin shifts the timing of both rhythms forward, a phenomenon called phase advancement, and that this shift is significantly greater than what happens with placebo.¹⁴ In practical terms, this means taking melatonin in the evening could cause your heart rate’s nightly dip to arrive earlier and potentially overlap with a period when you are still awake and upright, which is when a slow heart rate is most likely to cause symptoms like dizziness or lightheadedness.

This phase-shifting effect is distinct from the direct heart-rate-lowering mechanism discussed earlier. One acts acutely through parasympathetic activation, and the other acts over days by resetting your body’s internal timing. Both could contribute to an unexpectedly slow heart rate in certain people, but through different pathways and on different timescales. Someone who notices feeling lightheaded shortly after taking melatonin is likely experiencing the acute autonomic shift. Someone who feels fine initially but develops symptoms after days or weeks of use might be experiencing a cumulative chronobiological change.

Practical Considerations If You Take Melatonin

Most people taking standard doses of melatonin for sleep will never experience bradycardia, and the available evidence suggests the risk is genuinely low. The controlled studies show either no heart-rate change or a drop of a few beats per minute, well within what your heart does naturally over the course of a day. The risk factors worth paying attention to are straightforward:

• High doses: Taking 10 mg or more pushes blood levels far above what the body produces naturally and increases the likelihood of cardiovascular effects.
• Low baseline heart rate: If your resting heart rate is already in the low 50s or below, adding a parasympathetic stimulus leaves less margin before the rate drops into symptomatic territory.
• Heart-slowing medications: Beta-blockers, ivabradine, digoxin, and some calcium channel blockers all reduce heart rate through their own mechanisms. Combining them with melatonin’s autonomic effects could be additive.
• Supplement quality: Given the wild variability in melatonin content from one brand or lot to the next, choosing a third-party-tested product reduces the chance of inadvertently taking several times the intended dose.

If you have noticed a slow pulse, dizziness on standing, or unusual fatigue after starting melatonin, those are symptoms worth mentioning to a healthcare provider. The case report discussed at the beginning of this article is a useful template: the patient’s bradycardia resolved on its own once melatonin’s effects wore off, and no invasive treatment was needed. Stopping the supplement and monitoring is typically enough.