A 5mg dose of melatonin typically lasts four to five hours, with its strongest sleep-promoting effects occurring within the first one to three hours after taking it. The supplement’s half-life is short, around 20 to 50 minutes, meaning your blood levels drop by half in under an hour. But the sleepiness it triggers can linger well beyond that initial peak, especially at a dose this high.
How Quickly 5mg Peaks and Fades
Fast-release melatonin (the most common type sold over the counter) absorbs quickly and reaches peak blood concentration within about 45 minutes to an hour. From that peak, levels decline rapidly. Your body clears melatonin from the bloodstream with a half-life of roughly 20 to 50 minutes, so after two to three hours, most of the supplement has been metabolized by your liver.
That said, “cleared from the blood” and “no longer affecting you” aren’t the same thing. The sleepiness you feel can persist for up to five hours total because melatonin has already sent signals to your brain’s sleep-wake system. If you take 5mg at 10 p.m., expect noticeable effects until around 2 or 3 a.m. for most people.
Why 5mg Is a Large Dose
Your body naturally produces only about 0.1 to 0.5mg of melatonin per night. A 5mg supplement is at minimum ten times that amount, and potentially fifty times more than what your brain releases on its own. Research has shown that doses as low as 0.3mg can restore melatonin to levels seen in healthy young adults during nighttime, which is all that’s needed to support the sleep signal.
At 5mg, you’re flooding your system well beyond physiological levels. This doesn’t necessarily make it more effective for falling asleep, but it does mean the effects last longer and the chance of next-morning grogginess increases. Many sleep specialists suggest starting at 0.5 to 1mg and only increasing if that doesn’t work.
Immediate-Release vs. Extended-Release
The type of melatonin you took matters as much as the dose. Immediate-release tablets deliver the full 5mg into your bloodstream within the first hour, creating a sharp spike that fades relatively fast. This formulation works best for people who have trouble falling asleep but stay asleep fine once they drift off.
Extended-release formulations dissolve slowly, releasing melatonin over several hours. One small study found that a combined fast-release and extended-release formula kept melatonin levels elevated above baseline for nearly 10 hours in older adults. If your 5mg tablet is extended-release, its effects could stretch well into the following morning, making grogginess more likely.
Factors That Make It Last Longer or Shorter
Your liver does the heavy lifting when it comes to breaking down melatonin. Anything that affects liver function or the specific enzyme responsible for processing melatonin (called CYP1A2) will change how long 5mg sticks around.
- Age: Melatonin production naturally declines between ages 20 and 30 and continues dropping. Older adults also tend to metabolize supplements more slowly, so the same 5mg dose can produce stronger, longer-lasting effects in a 65-year-old than a 25-year-old.
- Liver health: People with liver disease clear melatonin much more slowly. In patients with cirrhosis, the half-life nearly doubles to about 100 minutes, and elevated levels can persist well into the next day.
- Caffeine: Caffeine is processed by the same liver enzyme as melatonin. When both are competing for that enzyme, melatonin clearance slows. Drinking coffee in the evening could paradoxically make your melatonin supplement last longer, though caffeine’s stimulant effects will counteract the sleepiness.
- Certain medications: The antidepressant fluvoxamine is a strong inhibitor of the enzyme that breaks down melatonin. If you take both, melatonin levels can stay elevated far longer than expected. Beta-blockers, on the other hand, can suppress your body’s own melatonin production, which changes the overall picture in a different way.
- Light exposure: Blue light (the kind from screens and LED bulbs, in the 460 to 480 nanometer range) is the strongest suppressor of melatonin. Scrolling your phone after taking melatonin can partially counteract its effects and shorten the perceived duration.
Next-Morning Grogginess
The so-called “melatonin hangover” is one of the main reasons people search for how long their dose will last. At 5mg, residual drowsiness the next morning is common, particularly if you took the supplement fewer than seven or eight hours before your alarm. The grogginess isn’t dangerous, but it can feel like waking up through fog, with slower reaction times and a pull to go back to sleep.
This effect is more pronounced with extended-release formulations, in older adults, and in people taking other sedating medications. If you consistently wake up groggy after 5mg, that’s a strong signal the dose is higher than you need. Cutting to 1mg or even 0.5mg often eliminates the hangover while still helping with sleep onset. Since melatonin isn’t FDA-regulated as a drug, there are no official maximum dose guidelines, but the research consistently suggests that lower doses mimic your natural rhythm more effectively than high ones.
Timing It Right
Because 5mg peaks in under an hour and its strongest effects last three to five hours, timing matters more than most people realize. Taking it 30 to 60 minutes before you want to fall asleep lines up the peak with your target bedtime. Taking it too early means the effects start fading before you’ve gotten deep into your sleep cycle. Taking it too late, say right as you get into bed, means you’ll be lying awake for 30 to 45 minutes waiting for it to kick in.
If you’re using melatonin to shift your sleep schedule (for jet lag or a changed work schedule), the timing window shifts too. For eastward travel or an earlier bedtime, taking it in the early evening can advance your body’s clock. For westward travel or a later bedtime, a small dose in the morning may help. In either case, the four-to-five-hour active window of a 5mg dose is the period during which your body’s internal clock is most responsive to the signal.