A 5 mg dose of melatonin typically lasts about four to five hours in your body. Melatonin has a short half-life of roughly 20 to 50 minutes in the bloodstream, meaning levels drop by half in under an hour. But the sleepiness-promoting effects extend beyond that window because melatonin works by signaling your brain’s sleep cycle, not by sedating you directly. Most people find that a 5 mg dose covers the first half of the night, with blood levels dropping to negligible amounts by early morning.
How Quickly It Peaks and Fades
Standard immediate-release melatonin tablets hit peak blood levels fast, usually within 30 to 60 minutes of swallowing the pill. From that peak, levels decline quickly. The elimination half-life ranges from about 40 minutes to 1.2 hours depending on the individual, so after two hours your blood concentration is already a fraction of what it was at its highest point. After four to five half-lives (roughly four to six hours total), the dose is essentially cleared from your system.
This matters practically because 5 mg is a relatively high dose. Your body naturally produces far less melatonin on its own each night, so 5 mg creates blood levels many times above what your brain would generate naturally. Even as those levels fall rapidly, they stay above your body’s baseline for several hours before fully clearing. That’s why the functional window of sleepiness tends to last longer than the raw half-life numbers suggest.
Why Only a Fraction of the Dose Reaches Your Blood
Oral melatonin has a bioavailability of roughly 10 to 35 percent. That means if you take a 5 mg tablet, only about 0.5 to 1.75 mg actually makes it into your bloodstream. The rest is broken down by your liver before it ever circulates. This first-pass metabolism is handled almost entirely by a single liver enzyme called CYP1A2, which is the same enzyme responsible for processing caffeine.
This is one reason people respond so differently to the same dose. If your liver is particularly efficient at this process, you might clear melatonin faster and feel its effects for a shorter window. If it’s slower, the effects may linger. Genetic variation in CYP1A2 activity is common, which helps explain why one person finds 5 mg too strong while another barely notices it.
Immediate-Release vs. Extended-Release
The type of melatonin tablet you take changes how long the effects last significantly. Immediate-release formulations dump all the melatonin into your system at once, creating a sharp spike that fades within a few hours. This works well if your main problem is falling asleep but you stay asleep fine once you’re out.
Extended-release (sometimes called slow-release or continuous-release) formulations spread the dose out over several hours, producing lower peak levels but maintaining them for longer. In pharmacokinetic studies, immediate-release melatonin reached peak blood concentrations roughly five times higher than continuous-release versions of the same dose, but the extended-release version maintained measurable levels further into the night. If you tend to wake up at 2 or 3 a.m., an extended-release formula will last longer in a practical sense. The NHS recommends slow-release melatonin tablets taken 30 minutes to two hours before bed, depending on whether the sleep problem is short-term or ongoing.
Factors That Change How Long It Lasts
Several things can speed up or slow down your body’s processing of melatonin:
- Caffeine timing. Caffeine competes for the same liver enzyme that breaks down melatonin. If you drink coffee in the evening, it can actually slow melatonin clearance, making the dose last longer and potentially contributing to morning grogginess.
- Smoking. Tobacco smoke is a potent activator of CYP1A2. Smokers tend to break down melatonin faster, which can shorten how long a dose lasts.
- Food. Taking melatonin with a meal, especially a high-fat one, slows absorption. This delays the peak but can also extend the overall duration of the effect.
- Body size. A 5 mg dose produces higher blood concentrations in a smaller person than a larger one, so the effects may last somewhat longer.
Age and Gender Differences
Research published in the American Journal of Physiology found that melatonin’s half-life does not differ significantly between younger and older adults, or between men and women. In young adults, the measured half-life was about 1.1 to 1.2 hours regardless of sex. In elderly subjects, the values were similar at roughly 0.9 to 1.1 hours. So age alone doesn’t meaningfully change how long a 5 mg dose sticks around in your system. What does change with age is how much melatonin your body produces naturally, which is why older adults are more likely to use supplements in the first place.
Morning Grogginess From 5 mg
The most common complaint with 5 mg is feeling groggy or foggy the next morning. This happens when melatonin levels haven’t fully cleared by the time your alarm goes off. If you take 5 mg at 10 p.m. and sleep until 6 a.m., that’s an eight-hour window, which is generally enough time for the dose to clear. But if you take it late, say at midnight, and wake at 6 a.m., residual effects are more likely.
Many sleep specialists consider 5 mg a high dose. Doses of 0.5 to 3 mg are often enough to shift sleep timing without the lingering effects. If you’re waking up groggy, a lower dose or an earlier timing (one to two hours before bed rather than right at lights-out) can help the melatonin do its job and clear your system before morning. The goal is to mimic your body’s natural melatonin curve, which rises in the evening and drops off well before dawn, not to maintain artificially high levels through the entire night.