Most of a day. It starts as your blood alcohol reaches zero, it is usually worst in the first few hours after that, and for most people it has largely cleared by the following evening.
Notice what that answer does not contain: a fixed number. The researchers who study this stopped using one on purpose, and the reason tells you more about your morning than any figure would.
When it starts, and why "the next day" is the wrong frame
The Alcohol Hangover Research Group's current definition describes a hangover as the combination of negative mental and physical symptoms that can follow a single drinking episode, "starting when blood alcohol concentration approaches zero."
An earlier version said "the day after." They removed it. If you drink at lunch, the hangover arrives that evening. The clock does not start at midnight. It starts when the alcohol runs out.
Which means the first thing that decides how long your hangover lasts is how long the alcohol took to leave — and that is arithmetic. Your liver clears about one standard drink an hour, give or take a good deal between people. After a heavy night, zero can land in the middle of the following afternoon, and the hangover starts then, not when you woke up feeling terrible. The full numbers are here.
Alcohol hangover — per the research definition, the negative mental and physical symptoms experienced after a single episode of drinking, beginning as blood alcohol approaches zero. There is no clinical cut-off for when it must end.
How long, honestly
There is no formally defined maximum. We looked for the "up to 24 hours" figure that circulates everywhere and could not find it stated as a defined endpoint by the research group.
What does exist is one study that measured it directly: an average of 18.4 hours from the last drink, roughly twelve hours from waking, with most people landing between 14 and 23. And it found something worth pausing on — total duration tracked how severe the hangover was and how much people slept, but not how much they drank.
So: most of a day is the honest summary, and what stretches it toward the long end is less about the drink count than about what the drink count did to your sleep and your clearance time.
What makes it last longer
Dose — but indirectly. More alcohol takes longer to reach zero, so the hangover starts later and finishes later. The duration study above did not find dose predicting length once it had begun; it moves the start line rather than the finish.
How fast you personally clear alcohol. In a study measuring breath alcohol across the morning, people with faster elimination rates reported milder hangovers — small sample, flagged by its authors as preliminary, but it points the same direction as the arithmetic above.
How much is still in you. Residual alcohol the morning after tracks nausea, fog and sleepiness closely, and the people who call themselves hangover-immune tend to have less of it on board. Some of the hangover is alcohol that has not left. Why some people seem to skip it entirely.
What you drank. In a double-blind crossover of 95 drinkers, bourbon produced worse hangovers than vodka at matched doses. Dark spirits carry more congeners. More on that here — including the catch, which is that next-day impairment was identical.
How you slept. Alcohol disrupts the second half of the night at doses lower than most people assume, and a hangover on four hours of broken sleep is a different animal from one on seven. What alcohol does to sleep.
Your genetics. Enzyme variants that slow the clearance of acetaldehyde — the flush reaction common in people of East Asian descent — plausibly extend things, though direct hangover-duration-by-genotype data are thin and we are not going to overstate it.
What does not make it last longer
Your age. This is the one everyone gets backwards. A study of 761 drinkers aged 18 to 94 found hangover severity and frequency decline with age, and the relationship held after controlling for how much people drank. What changes with age is dose sensitivity and how much slack the next day has, not the hangover itself. We went through that properly.
The "two-day hangover" — what it is and is not
It is not a clinical entity. We could find no peer-reviewed definition of a second-day phase, no study measuring one, and no mention of it in the research group's literature. It is a phrase from wellness media.
What people are describing is real, though, and it has an explanation that does not need a new category: a genuinely large dose that took most of a day to reach zero, so the hangover started late and ran late; sleep debt stacking across two nights; and the ordinary compounding of a day spent recovering badly. We wrote that up separately, without inventing a syndrome.
When it is not a hangover
If symptoms are lasting well into a second day, getting worse rather than better, or look like any of the following, it is worth recognising that a hangover is not the only thing this can be.
The tell is direction. A hangover eases across the day. Withdrawal — in someone whose body has adapted to regular heavy drinking — gets worse, starting within a day of the last drink and escalating from tremor and sweating toward hallucinations or seizures. And alcohol poisoning is an overdose in progress, not an aftermath: someone who cannot be woken, is breathing slowly or irregularly, or has clammy or bluish skin needs emergency services, not sleep.
The full red-flag list, with the NIAAA and clinical timelines, is here.
What shortens it
Nothing shortens the alcohol's exit. What you can do is stop making the tail worse.
- Sleep normally the following night. No 4pm nap, no alarm heroics. Sleep debt is the part that carries over.
- Eat, and put salt in it. What actually helps.
- Replace fluid and minerals — the part of the morning that is genuinely replaceable.
- Daylight and gentle movement. A walk beats a dark room.
- Do not have a drink. It resets the clock. Why hair of the dog defers rather than resolves.
Frequently asked questions
How long does a hangover last? Most of a day, starting from when blood alcohol reaches zero — which, after a heavy night, can be well into the afternoon. There is no formally defined maximum.
Why is my hangover lasting two days? Most often because the dose was large enough that the alcohol took most of day one to clear, so the hangover started late, and because sleep debt stacked across two nights. A defined "two-day hangover" does not exist in the research.
Do hangovers last longer as you get older? No. Measured severity and frequency decline with age. What rises is the blood alcohol a given round produces, because body water falls.
Can you speed up a hangover? You cannot speed the alcohol leaving. You can avoid extending the tail: sleep, food, fluid with salt, daylight, and no more alcohol.
How long does a hangover last after a night of heavy drinking? Longer, in proportion to the dose — because zero arrives later and the hangover starts from there. Eight drinks can put zero at mid-afternoon.
When should I worry? When the direction is wrong — getting worse instead of better — or when there is escalating tremor, confusion, a seizure, or a person you cannot rouse. None of those belongs to a hangover.
Where Snake Oil fits
A hangover runs on the alcohol's schedule, not ours, and no drink mix changes it.
What it does have is a replaceable component. Of everything on the timeline, the fluid and minerals are the one input you put back rather than wait out. One stick, 16 ounces of water: 370mg sodium, 5g carbohydrate so the sodium is retained, and the potassium, magnesium, zinc, B vitamins, C, E and glutamine-led amino acids that ride with them.† Before bed is when most people use it, for reasons explained here.
The rest is sleep and time. We would rather say so.
†These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
Related reading: How long does alcohol stay in your system? · Two-day hangover: why you still feel off