We get hungover because alcohol disrupts multiple systems in the body at once—fluids, blood sugar, sleep, gut, and immune function—and the “payback” shows up when the alcohol level falls and the brain and body try to rebalance.

What a hangover actually is

A hangover is a cluster of symptoms that appear as blood alcohol levels fall back toward zero: headache, fatigue, nausea, anxiety, light sensitivity, and more. It typically peaks the morning after heavy drinking and can last up to 24 hours or longer depending on how much you drank, your body size, and other factors.

Main reasons we get hungover

Think of a hangover as several mini problems hitting at once rather than one single cause.

  • Dehydration and electrolyte loss
    Alcohol blocks vasopressin (an anti‑diuretic hormone), so your kidneys dump more water and electrolytes, leading to thirst, dizziness, and headache. Even if you drink water later, your body has already had a period of relative “dryness” that your brain does not enjoy.
  • Gut irritation and nausea
    Alcohol directly irritates the stomach lining, increases acid production, and can speed up or slow digestion, which is why you may feel nausea, vomiting, or diarrhea. This irritation also makes it harder for your body to absorb nutrients and can leave your stomach feeling raw the next day.
  • Blood sugar swings
    Your liver works on breaking down alcohol instead of keeping blood sugar stable, so glucose levels can drop, causing shakiness, fatigue, and irritability. If you did not eat much while drinking, that low blood sugar effect tends to be worse.
  • Sleep disruption
    Alcohol can help you fall asleep faster but leads to fragmented, lower‑quality sleep and less REM sleep later in the night. Even if you spend many hours in bed, the poor sleep architecture leaves you feeling foggy and exhausted.
  • Immune and inflammatory changes
    After a heavy drinking session, the immune system releases inflammatory molecules (cytokines) that can contribute to headache, malaise, and “flu‑ish” feelings. This mini inflammatory response helps explain why you can feel mentally slow and physically achy even after rehydrating.
  • Acetaldehyde and other toxic by‑products
    Alcohol is broken down into acetaldehyde, a reactive compound that is more toxic than ethanol itself and is linked to many unpleasant symptoms. Your body eventually converts acetaldehyde into less harmful substances, but during the process you can feel flushed, nauseated, and headachy.

Why some hangovers feel worse

Not everyone gets hangovers the same way, even at similar drinking levels.

  • Type and amount of alcohol
    Darker spirits like bourbon or brandy contain more congeners (fermentation by‑products) that can intensify hangovers compared with clearer drinks like vodka. Larger total alcohol intake and drinking faster both strongly increase hangover risk and severity.
  • Body size, sex, and genetics
    People with lower body weight or lower body water content reach higher blood alcohol levels from the same number of drinks. Genetic differences in enzymes that process alcohol and acetaldehyde also influence how intense hangover symptoms become.
  • Food, sleep, and other drugs
    Drinking on an empty stomach allows alcohol to hit the bloodstream quickly, making hangovers more likely. Smoking or using other substances alongside alcohol is also associated with stronger hangovers.

Why “hair of the dog” feels like it works (but doesn’t)

Some people feel a bit better after having another drink the next morning because alcohol can temporarily dull the perception of symptoms. However, this just delays the underlying recovery process and can increase the overall strain on the body and the risk of unhealthy drinking patterns.

TL;DR: We get hungover because alcohol temporarily throws off fluid balance, irritates the gut, disrupts sleep and blood sugar, and triggers inflammatory and metabolic changes; when the alcohol wears off and the body tries to reset, those combined effects are experienced as a hangover.

Information gathered from public forums or data available on the internet and portrayed here.