What causes a hangover?

Your phone lights up with blurry screenshots, half finished voice notes, and a mysterious food delivery receipt from 2.43 a.m. Your mouth feels dry, your head feels too loud for how quiet the room is, and even sitting up feels like a small workout. You did not just wake up with “a bit of discomfort”. You woke up in the middle of a complex chain reaction that alcohol started in your body hours ago. A hangover is not just punishment for a wild night. It is a full body response that involves your brain, liver, gut, immune system, sleep cycle, and even your emotions the next morning. When you drink, alcohol moves quickly from your stomach and small intestine into your bloodstream. Once it is in your blood, it reaches almost every organ that has a decent blood supply. Your liver is at the center of this story. It acts like the main moderator trying to keep things under control. Its job is to break down alcohol into other substances that your body can eventually clear out. First, alcohol is converted into a compound called acetaldehyde. Then acetaldehyde is turned into acetate, which is less toxic and easier for the body to process and remove.

The trouble is that acetaldehyde is not a harmless middle step. It is highly reactive and irritating. While it is circulating in your system, it can damage cells and disrupt normal processes. If you have been drinking quickly, or if your body metabolizes alcohol more slowly, acetaldehyde has more time to build up. That toxic middle stage is one of the main reasons you can feel so unwell the next day. So it is not just the alcohol you drink that matters. It is also what your body turns it into, and how fast that happens.

On top of that, alcohol quietly works against your fluid balance. Normally, your body produces a hormone called vasopressin that helps your kidneys hold on to water. Alcohol suppresses this hormone, which means your kidneys let more water go than they usually would. You make more trips to the bathroom, even if you are not drinking much water between drinks. By the time you go to bed, you have probably lost more fluid than you realize. Dehydration does not just mean you feel thirsty. It affects your blood volume, which can influence your heart rate and blood pressure. It contributes to headaches because the balance of fluid around your brain changes. It dries out your mouth and eyes. So when you wake up, you are not just tired from staying out late. You are also slightly dried out and your body chemistry has shifted.

The sleep you got was likely not very helpful either. Many people think passing out after drinking is the same as getting a deep, heavy sleep. In reality, alcohol disrupts normal sleep architecture. It might help you fall asleep more quickly, but it tends to fragment your sleep and reduce the amount of deeper, restorative stages you pass through. It can also interfere with REM sleep, the stage linked to dreaming, emotional processing, and memory. You might wake up several times during the night without remembering it, or sleep lightly without entering the deeper phases your brain and body need. That is why you can spend eight hours in bed but still wake up feeling as if you barely slept. Some of what you call a hangover is actually a mix of sleep debt and uncompleted overnight “maintenance” in your brain.

Your immune system quietly joins this chaotic scene too. After heavy drinking, your body can respond as if it is facing a minor threat. It releases inflammatory molecules that are meant to protect you in other situations, such as infection or injury. In the context of a night out, these chemicals simply add to how unwell you feel. Inflammation can bring on body aches, sensitivity to light and sound, and a general sense of weakness that feels similar to the early stages of a flu. That is partly why a bad hangover feels like more than just being tired or having a headache. Your body is responding on several levels to what it sees as a stress event.

Your digestive system is also under pressure. Alcohol irritates the lining of your stomach and intestines, increases acid production, and can slow down how quickly your stomach empties. It can disturb the normal muscular movements that help push food through your digestive tract. It may also affect the balance of bacteria in your gut, which usually help with digestion and other important functions. This combination can easily turn into nausea, vomiting, cramps, or that familiar queasy feeling that does not match how much you have actually eaten. If you drank on an empty stomach, the irritation is often stronger. If you paired your drinks with very greasy or heavy food, your gut has even more work to do the next day. When your stomach feels unsettled and unpredictable, that discomfort is a visible sign of the irritation and disruption happening inside.

Blood sugar swings add another layer to the experience. Your body usually keeps your blood sugar within a relatively narrow range. Alcohol can interfere with this balance, especially if you have not eaten enough. It can affect how your liver releases stored glucose into your bloodstream, leading to lower blood sugar several hours after drinking. Low blood sugar can make you feel shaky, weak, light-headed, and desperately hungry. It also affects your mood. Your brain relies on a steady supply of glucose, so when that supply dips, you may feel more emotional, irritable, or overwhelmed than usual. These emotional ripples combine with lack of sleep, dehydration, and inflammation, turning a simple morning into something that feels harder than it should be.

Then there are congeners, the extra substances present in alcoholic drinks besides ethanol itself. These are compounds produced during fermentation and aging that contribute to a drink’s taste, smell, and color. Darker drinks like whiskey, brandy, and red wine generally contain more congeners, while clearer spirits such as vodka usually contain fewer. Some research suggests that drinks with higher levels of congeners can make hangovers more intense for many people. So it is not only how much you drink that matters, but also what you drink. A night that jumps between sugary cocktails, dark spirits, and wine is especially confusing for your system. Each type of drink brings its own extras that your body then has to process.

Personal biology also plays a big role. Two people can drink the same amount and wake up in very different states. This is not only about “being able to hold your liquor”. It is about genetics and overall health. Some people have genetic variations in the enzymes that break down alcohol and acetaldehyde. If your enzymes work more slowly at any step, alcohol or its byproducts may linger longer in your system, making you more sensitive to the aftereffects. People from some ethnic backgrounds are more likely to have these variations. That familiar flushing of the face, pounding heartbeat, or sudden warmth during drinking can be a sign that your body is processing alcohol differently. Body size, sex, hormone levels, medications, and stress also influence how you feel after a night out. A smaller person usually reaches a higher blood alcohol level from the same number of drinks. Someone who was already tired or anxious before the first drink may find the next morning especially difficult.

On top of the physical symptoms, there is a mental and emotional side that many people know too well. Alcohol changes the activity of neurotransmitters, the chemicals that help nerve cells communicate. While you are drinking, it tends to increase processes that calm the nervous system and reduce inhibition. This can feel like relaxation, confidence, or social ease. Later, as your blood alcohol level falls, your brain can swing back in the opposite direction. It may become more excitable and sensitive, creating a rebound effect. That rebound can show up as anxiety, racing thoughts, a vague sense of dread, or intense embarrassment about things you said or did. Many people call this hangxiety. It is not simply imagined. It reflects real changes in your brain chemistry, stacked on top of poor sleep, physical discomfort, and blood sugar changes. Your body is sending scattered distress signals and your mind is doing its best to make sense of them, often by assuming the worst.

If you zoom out from all these details, a clearer picture appears. A hangover is the combined effect of many small disruptions that have not yet settled down by the time you wake up. Alcohol and its byproducts have irritated your brain and gut. Your liver has been working all night to turn toxic substances into safer ones. Your kidneys have let go of more water than usual, nudging you toward dehydration. Your sleep has been shallow and broken. Your immune system has responded with low level inflammation. Your blood sugar may be on the low side. Your nervous system is swinging between wired and exhausted. None of these pieces alone might be enough to ruin your morning, but together they build the familiar foggy, pounding, unsettled state you recognize as a hangover.

Understanding what causes a hangover does not magically make it pleasant, but it does change the story. It is not a mysterious punishment or a moral judgment dropping from the sky. It is your body trying to stabilise itself after several of its systems were pushed at the same time. Every drink has an effect that goes beyond the hour you are holding the glass. The next time you are out with friends, ordering another round and laughing at the group chat, you might still decide to stay a little longer or try that extra cocktail. But at least you will know that the rough morning after is not random. It is your very human biology presenting the bill for a night your body never got to vote on.