To understand how long a hangover lasts, it helps to know what a hangover is: the mental, emotional and physical symptoms which an individual experience after imbibing more alcohol than the liver can metabolize. The longer the body takes to break down the booze, the longer and more intensely you’ll feel the pain. 

Typically, the classic hangover lasts for about 24 hours, but depending on how much you had to drink, they can last up to 72 hours (three-day hangovers, anyone?). Hangovers begin within several hours after you stop drinking alcohol when blood alcohol concentration (BAC) falls. Symptoms of a hangover usually peak about the time BAC is zero and may continue for up to 24 hours or more thereafter.  There are also at least 5 factors that influence the intensity and duration of a hangover.

#1. Too much alcohol

The strength and length of your hangover often are directly related to how much you drank.1 In other words, the more alcohol you consumed, the longer it’s going to take your body to process it and lower your blood alcohol content (BAC), which means the more severe and longer your hangover will last. If you want to avoid a lengthy hangover, the best thing you can do is to drink less.

#2. Age

As we get older, our cells age, and we might not be able to process toxins as we did when we were younger. Although there’s a lot of evidence that hangovers really do worsen with age, there is very little concrete evidence as to why this is.2 Possible explanations are:

  • Our body composition changes with time. Older individuals have less total body water thereby decreasing the volume in which substances, such as alcohol, can circulate. This can lead to a higher blood alcohol concentration (BAC).
  • Enzymes are responsible for metabolizing alcohol to help clear it out of our system, so this could mean it ends up being in the body for a longer time, thus prolonging a hangover. One hypothesis is that we have fewer liver enzymes (ADH and ALDH) or that these enzymes do not work as efficiently the older we get.

One study found that as mice got older, they had less glutathione, an antioxidant that helps detoxify alcohol.It’s possible that something similar happens in people. 

#3. Hydration

Alcohol is also a natural diuretic. This means that alcohol stimulates the body to excrete vitamins, minerals, and salt through the kidneys, which could lead to dehydration and chemical imbalances. Dehydration itself is one of the main causes of hangovers. Common symptoms of dehydration caused by alcohol include dry mouth, thirst, dizziness, and headache. The more dehydrated you are, the worse your hangover will be. Stay hydrated by alternating between drinking alcoholic beverages and drinking plenty of water. 

#4. Sleep

A lack of sleep can seriously compound the severity of hangover symptoms.4 Alcohol may seem to help you sleep, as it helps induce sleep, but overall it is more disruptive to sleep.

A review of 27 studies shows that alcohol consumption worsens sleep quality.5 According to this review, alcohol does allow healthy people to fall asleep quicker and sleep more deeply for a while, but it reduces rapid eye movement (REM) sleep. This is because your body is rebounding from the depressive effect of alcohol, disturbing your normal sleep rhythm, so you won’t get as much deep, quality sleep.

The bottom line is: the more you drink, the worse you sleep, and the worse you feel the day after. If you’re dealing with a horrific hangover, try taking a nap to feel better.

#5. What you Drink

The type of alcohol you consume also contributes to how you’re going to feel the next day. In alcoholic beverages, there are impurities called congeners produced during fermentation, which are responsible for some of the taste, aroma, and color of what you drink. These impurities are not the sole cause of a hangover, yet they do seem to contribute in some manner to the severity.6,7

Darker colored alcoholic drinks, such as whiskey, brandy, bourbon, and red wine have higher congener concentrations than lighter drinks such as vodka, gin, and white wine. A recent study about congeners and hangovers concluded that brandy and red wine are at the top of the list for causing severe hangovers. Last on the list is vodka, causing the fewest symptoms.8

Consider opting for light drinks over dark ones—they tend to have fewer congeners. Heavy drinking, no matter what kind, will probably make you ill the next morning.
  
Sources: 
  1. http://annals.org/aim/fullarticle/713513/alcohol-hangover?resultClick=3 
  2. https://www.ncbi.nlm.nih.gov/pubmed/24033827 
  3. https://www.ncbi.nlm.nih.gov/pubmed/19398231 
  4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5499928/ 
  5. https://www.ncbi.nlm.nih.gov/pubmed/23347102 
  6. https://www.ncbi.nlm.nih.gov/pubmed/20712591 
  7. https://researchbank.swinburne.edu.au/items/d0e3fd65-2acc-4c66-b074-b1e2ad48f2b9/1/ 
  8. http://student.bmj.com/student/view-article.html?id=sbmj0206184