Melatonin is one of the most widely used sleep supplements, and while it’s generally considered safe for short-term use, it does come with a real list of side effects. These range from next-day grogginess and headaches to less obvious concerns like hormonal shifts, blood sugar changes, and interactions with common medications. The risks increase with higher doses, longer use, and in certain populations, especially children.
Common Side Effects
The most frequently reported side effects of melatonin are relatively mild. Daytime sleepiness tops the list, which makes sense given the supplement’s purpose. Headaches, nausea, stomach pain, and dizziness are also common. Some people experience dry mouth, dry or itchy skin, and unexplained aches in their arms or legs. Irritability and restlessness round out the typical complaints.
Less common but still documented effects include temporary feelings of depression and mild anxiety. These mood-related side effects tend to resolve once you stop taking the supplement, but they’re worth knowing about if you’re already managing a mood disorder.
Daytime Grogginess and the “Hangover” Effect
One of the most common complaints is feeling foggy or drowsy the next morning. Research on a 6 mg slow-release formulation found that melatonin-induced sleepiness first appeared about 1.25 hours after taking it and persisted for more than 4 hours. In that study, subjective sleepiness briefly dipped around the 4-hour mark but then returned at 5 and 6 hours post-dose.
The good news: despite that lingering sleepy feeling, melatonin didn’t actually impair psychomotor performance in the same study. Participants felt tired but could still perform tasks normally, which sets melatonin apart from prescription sleep medications that measurably slow reaction times and coordination. Still, feeling subjectively drowsy can affect your day, especially if you’re driving or operating machinery in the morning.
Vivid Dreams and Nightmares
Strange or intense dreams are a well-known side effect. Melatonin increases the amount of time you spend in REM sleep, the sleep stage most associated with vivid dreaming. It also triggers the release of a protein called vasotocin that further regulates REM sleep. More melatonin means more vasotocin, which means more REM time, and with that comes a higher chance of unusually vivid or disturbing dreams. Night sweats often accompany these sleep disturbances.
For most people this is just an annoyance, but if you’re prone to nightmares or have PTSD-related sleep disturbances, the amplified dream activity could be disruptive enough to outweigh the benefits.
Effects on Hormones
Melatonin doesn’t just regulate your sleep-wake cycle. It interacts with reproductive hormones in ways that matter, particularly for women. At high doses, melatonin combined with progesterone can suppress ovulation. It also modulates progesterone production in the cells that support egg development and early pregnancy. One study found that taking 3 mg of melatonin nightly raised concentrations inside ovarian follicles by fourfold.
The relationship isn’t entirely negative. In women with polycystic ovary syndrome (PCOS), melatonin supplementation for 3 to 6 months actually improved menstrual regularity and helped normalize androgen levels. But for women actively trying to conceive or those without PCOS, the hormonal effects of supplemental melatonin are largely uncharted and potentially disruptive.
Blood Sugar and Diabetes Risk
Melatonin temporarily raises blood sugar by reducing insulin release. It binds to receptors on the insulin-producing cells of the pancreas, signaling them to slow down. This effect is most pronounced if you take melatonin close to a meal, especially a high-carbohydrate one. For people with glucose intolerance or type 2 diabetes, this short-term spike can be clinically meaningful.
Interestingly, the long-term picture may look different. Some studies suggest that extended melatonin use can modestly improve insulin sensitivity and lower average blood sugar over time in people with type 2 diabetes. But the short-term risk of blood sugar spikes means timing matters. If you take diabetes medications, adding melatonin without adjusting your routine could create unpredictable glucose swings.
Drug Interactions
Melatonin interacts with several common medication classes in ways that can cause real problems:
- Blood thinners and anti-clotting supplements: Melatonin may increase bleeding risk when combined with anticoagulants or anti-platelet drugs.
- Seizure medications: Melatonin can reduce the effectiveness of anticonvulsants, potentially increasing seizure frequency. This is especially concerning in children with neurological conditions.
- Blood pressure medications: Melatonin may worsen blood pressure control in people already on blood pressure drugs.
- Sedatives and CNS depressants: Combining melatonin with other sedating substances creates an additive effect, increasing drowsiness beyond what either would cause alone.
- Diabetes medications: As noted above, melatonin’s effect on insulin release can interfere with blood sugar management.
Concerns for Children
Pediatric melatonin use has surged, and so have accidental ingestions. CDC data from 2012 to 2021 showed that among reported cases of children ingesting melatonin, over 81% of clinical effects involved the central nervous system (excessive drowsiness, lethargy), about 10% involved gastrointestinal symptoms, and roughly 2.5% involved cardiovascular effects.
The puberty question is the one parents most often worry about. In normal development, melatonin levels gradually decline before the onset of puberty, and there’s a theoretical concern that supplementing melatonin could delay that natural hormonal shift. A systematic review published in The Lancet found that three studies showed little or no effect on pubertal development after 2 to 4 years of treatment, but one study flagged a potential delay with treatment lasting more than 7 years. The evidence is rated very low certainty, meaning the risk may depend heavily on how long a child takes it.
What’s Actually in the Bottle
Because melatonin is sold as a dietary supplement in most countries, it isn’t held to the same manufacturing standards as prescription drugs. This creates a real accuracy problem. One widely cited analysis of supplements sold in Ontario, Canada found that about 71% of products didn’t contain the amount of melatonin listed on the label (within a 10% margin). The variation between lots of the same product reached as high as 465%. A more recent European study found that 24% of legally sold melatonin supplements contained less than 80% of the labeled dose, while some contained triple the permitted amount.
Perhaps more alarming, the Canadian study found serotonin (a breakdown product of melatonin) in 26% of supplements at levels considered potentially clinically significant in children. This introduces the risk of serotonin toxicity, an entirely separate danger that has nothing to do with melatonin itself. The inconsistency means you may be taking far more or far less melatonin than you think, and possibly other compounds you didn’t sign up for.