A 5mg dose of melatonin stays in your system for roughly 5 hours. Melatonin has a short half-life of 20 to 60 minutes, meaning your body eliminates half of it within that window. After four to five half-lives, the supplement is essentially cleared from your bloodstream. The exact timeline depends on the formulation you took, your age, and what else is in your system.
How Your Body Processes a 5mg Dose
When you swallow an immediate-release 5mg melatonin tablet, it reaches peak concentration in your blood within about 50 minutes. At that point, your body is already working to break it down. The liver processes melatonin using a specific enzyme, and only a small fraction of the original dose actually makes it into your bloodstream (melatonin has notoriously low bioavailability, often around 15%).
From peak concentration, here’s roughly what the decline looks like with a 40-minute half-life:
- 40 minutes after peak: ~2.5mg equivalent remains
- 80 minutes after peak: ~1.25mg remains
- 2 hours after peak: ~0.6mg remains
- 3–4 hours after peak: trace amounts remain
By about 5 hours after you took the pill, melatonin levels in your blood have returned to baseline. This is why people who take melatonin sometimes wake up in the middle of the night: the supplement wears off well before morning.
Extended-Release Changes the Timeline
If you took an extended-release or sustained-release formulation, the math changes significantly. These tablets dissolve slowly, releasing melatonin over several hours instead of all at once. A sustained-release 5mg capsule reaches its peak concentration at about 1.25 hours, compared to under an hour for immediate-release.
More importantly, the tail end is much longer. One study of older adults taking a combined fast-release and extended-release melatonin found that blood levels remained elevated above baseline nearly 10 hours after the dose. So if your bottle says “extended release,” “sustained release,” or “time release,” you can expect melatonin to stay active in your system roughly twice as long as the standard 5-hour window.
Why It Clears Faster or Slower in Some People
Melatonin is broken down in the liver by an enzyme called CYP1A2. Anything that speeds up or slows down this enzyme will change how long melatonin lingers in your body.
Things that make melatonin clear faster include smoking (cigarette smoke substantially increases CYP1A2 activity), grilled meat, and cruciferous vegetables like broccoli and Brussels sprouts. If you’re a regular smoker, melatonin may wear off noticeably sooner than the typical 5-hour window.
On the other hand, certain medications slow the enzyme down, which means melatonin hangs around longer and hits harder. Birth control pills containing ethinyl estradiol are a common one. The antibiotic ciprofloxacin and the antidepressant fluvoxamine also inhibit this enzyme. If you’re taking any of these, a 5mg dose could stay in your system well beyond 5 hours and produce stronger drowsiness than expected.
Caffeine is processed by the same liver enzyme, so heavy coffee consumption can create a bottleneck. Your body essentially has to choose between breaking down caffeine and breaking down melatonin, which can extend the effects of both.
Does Age Matter?
It’s a common assumption that older adults metabolize melatonin more slowly, but the evidence is less clear-cut than you might expect. Research published in the American Journal of Physiology found no significant difference in the duration of melatonin secretion between young and older adults, with both groups showing secretion windows of roughly 7.5 to 8.5 hours for their natural melatonin. That said, older adults often have lower baseline melatonin production, which is partly why supplemental doses can feel more potent. Liver function also tends to slow with age, which can modestly extend clearance time.
Why 5mg May Be More Than You Need
A 5mg dose is actually quite high relative to what your body produces naturally. Your pineal gland releases roughly 0.1 to 0.8mg of melatonin per night. Supplemental doses of 0.5 to 1mg are often enough to shift your sleep timing, and they clear your system faster, reducing the chance of next-morning grogginess.
At 5mg, your blood melatonin levels spike far above physiological range. While melatonin is not considered dangerous at this dose, the excess can leave you feeling groggy or foggy the next morning, especially if you took it late or if your metabolism runs on the slower side. If you find yourself dragging in the morning, the dose may simply be lingering longer than your sleep window requires. Trying a lower dose, or switching to an immediate-release formulation, often solves the problem.