A 5mg dose of melatonin typically lasts four to five hours. That’s the window during which blood levels remain elevated enough to promote sleepiness, and it’s also how long experts recommend waiting before driving or using machinery after taking it. The actual duration can shift depending on whether you took an immediate-release or sustained-release formula, your individual metabolism, and what else you consumed that day.
How Melatonin Moves Through Your Body
After you swallow a standard immediate-release melatonin tablet, blood levels peak in about 50 minutes. From that peak, the concentration drops quickly. The half-life of immediate-release melatonin is roughly one hour, meaning half the dose is cleared from your bloodstream every 60 minutes or so. Within four to five hours, levels have typically fallen back to baseline.
This is notably faster than many other supplements people take at bedtime. Melatonin doesn’t recirculate through your liver the way some compounds do, so once your body breaks it down, it’s gone. That short window is why some people find melatonin helps them fall asleep but doesn’t keep them asleep through the night.
Immediate-Release vs. Sustained-Release
The type of melatonin you bought matters more than most people realize. A pharmacokinetic study comparing the two formulations found that sustained-release melatonin has a half-life of about 5.1 hours, roughly five times longer than the 1-hour half-life of immediate-release. That difference changes the entire experience.
With immediate-release, blood levels spike higher and faster, peaking around 50 minutes, then drop sharply. Most of the melatonin is gone within a few hours. Sustained-release capsules peak a bit later (around 75 minutes) and at a lower concentration, but they maintain meaningful blood levels for up to eight hours. In the four-to-eight-hour window after taking them, sustained-release formulas delivered 1.4 to 3.1 times more melatonin in the bloodstream compared to immediate-release.
If your problem is falling asleep, immediate-release is designed for that. If you’re waking up at 2 or 3 a.m. and can’t get back to sleep, a sustained-release formula will keep working deeper into the night. Check the label on your bottle, because “melatonin 5mg” could be either type, and the duration of effect is dramatically different.
Why 5mg May Last Longer for Some People
Individual variation in melatonin metabolism is surprisingly large. Your liver processes melatonin primarily through a specific enzyme pathway, and the speed of that pathway varies from person to person based on genetics. Some people are fast metabolizers who clear the supplement quickly, while others process it more slowly and feel groggy the next morning.
Caffeine uses the same liver pathway as melatonin. In a study of 12 healthy adults, taking 200mg of caffeine (roughly one large cup of coffee) alongside melatonin more than doubled the peak blood concentration and increased overall melatonin exposure by 120%. For people with a particular genetic variant, the peak concentration tripled. Interestingly, caffeine didn’t change how quickly melatonin was eliminated. Instead, it caused the body to absorb more of the dose, making the effects stronger rather than longer.
Certain prescription medications have an even more dramatic effect. One antidepressant that inhibits the same enzyme pathway increased melatonin exposure by 17-fold and peak levels by 12-fold. If you’re taking any medication that affects liver enzymes, melatonin could hit much harder and potentially last longer than expected.
Timing Your Dose
Because melatonin peaks within about an hour, taking it 30 to 60 minutes before you want to fall asleep gives it time to build up in your bloodstream right when you’re getting into bed. Taking it too early means levels may already be declining by the time you turn off the lights.
If you’re using melatonin to shift your sleep schedule (for jet lag or shift work, for example), timing matters even more than dose. The American Academy of Sleep Medicine recognizes strategically timed melatonin as a treatment option for jet lag and shift work disorder, but does not recommend it for chronic insomnia in adults. The goal in those cases is to use melatonin as a signal to reset your internal clock, not as a sedative.
Why You Might Still Feel It in the Morning
At 5mg, you’re taking a dose that produces blood levels far above what your body generates naturally (your pineal gland releases the equivalent of roughly 0.1 to 0.3mg per night). Even though the supplement clears within four to five hours for most people taking immediate-release, a 5mg dose can leave some individuals feeling foggy or drowsy into the morning, especially slower metabolizers or anyone who took it too late at night.
If you find yourself groggy the next day, the fix is usually to take a lower dose or take it earlier in the evening. Many sleep researchers suggest that 0.5 to 3mg produces effects closer to what your body naturally does, and for a lot of people, a smaller dose works just as well without the morning hangover. The four-to-five-hour active window doesn’t change much with dose, but the intensity of the effect does.