You wake up with a pounding head, a queasy stomach, and a foggy brain. The easy explanation is dehydration. But according to several doctors, what’s really going on is quite a bit more complicated than that.
“Hangovers are highly individualized and subjective between different patients,” says Dr. Sumona Bhattacharya, a gastroenterologist and assistant professor of medicine at the George Washington University Medical Faculty Associates. Your weight, sex, and metabolism all play a role in how you feel the next morning.
And here’s something worth knowing: about 20 percent of the population is naturally “hangover resistant.” Scientists have identified this group. If you’re not in it, you’re in very good company.
Six Reasons You Feel So Rough
Dr. Alfred F. Tallia, a professor and chair of the Department of Family Medicine and Community Health at the Rutgers University Robert Wood Johnson Medical School, puts it plainly. “There are many factors involved with producing a hangover,” he says, including changes happening on a cellular level and mineral imbalances.
Here is what doctors say is actually happening in your body.
1. Your Liver Makes a Toxic Byproduct
When you drink, your liver breaks alcohol down into a compound called acetaldehyde. This is where a lot of the misery starts. “Acetaldehyde is a toxic compound. It causes nausea, flushing, and headache,” says Dr. Jonathan Avery, vice chair for addiction psychiatry at Weill Cornell Medicine and New York-Presbyterian.
The more you drink, the more acetaldehyde your body produces. Your liver uses an enzyme called aldehyde dehydrogenase to break it down. “If your body does not have good levels of the enzyme, the acetaldehyde hangs around and causes havoc,” says Dr. Tallia. Some people are simply more sensitive to this than others.
2. You Lose More Than Just Water
Yes, dehydration is real — but it’s just one piece. Alcohol has a diuretic effect, meaning it pushes water and electrolytes out through the kidneys, says Dr. Kelly Krisna Johnson-Arbor, a toxicologist at MedStar Health.
Electrolytes are minerals like sodium and magnesium that your body needs to function. When those drop, you feel it. “That can lead to fatigue and weakness,” says Dr. Avery.
3. Your Immune System Fires Up
Your body treats alcohol like an intruder. A 2020 study found that certain proteins called cytokines — released by the immune system to regulate inflammation — increased in people several hours after drinking.
“This inflammatory response may also impact the development of a hangover,” says Dr. Johnson-Arbor. That inflammation explains the headaches, body aches, and mental fog so many people describe. It is your immune system doing its job, whether you asked it to or not.
4. Your Brain Chemistry Swings Back
Alcohol quiets a brain chemical called glutamate, which plays a key role in memory and learning. When the alcohol wears off, glutamate levels bounce back fast. “That gives you a jittery, anxious, irritable feeling,” says Dr. Avery. This rebound effect is a real and recognized part of the hangover experience.
5. Your Sleep Was Not as Good as It Felt
Alcohol may help you drift off faster, but research shows it disrupts REM sleep — the deep, restorative stage responsible for emotional regulation and memory. “The sleep that occurs tends to be fragmented or disrupted,” says Dr. Bhattacharya. So even if you slept a full eight hours, you likely didn’t get the rest your body actually needed.
6. Darker Drinks Bring Extra Baggage
Here is one that surprises many people. Fermentation and aging produce chemical byproducts called congeners — substances like methanol, acetone, and tannins. Darker spirits and aged wines tend to have higher concentrations of them.
“These substances are associated with increased oxidative stress and inflammation,” says Dr. Johnson-Arbor. “Because congeners promote inflammation, they may also impact hangover development.”
The One Thing Doctors Agree On
Despite knowing all of this, doctors say there is still no proven cure for a hangover. “There are no known cures for a hangover aside from the avoidance of alcohol consumption,” says Dr. Johnson-Arbor.
How much alcohol affects you also comes down to your personal genetics, according to research. Everyone’s threshold is different. Dr. Tallia’s straightforward advice: if you want to avoid a rough morning, keep your intake small. “Consume alcohol in relatively small doses if you want to avoid the common hangover,” he says.
So now you know it’s not just the water you forgot to drink. There is quite a lot happening in your body — and most of it starts long before you wake up.