The pounding head. The parched mouth. And, worst of all, the crushing sense of anxiety as you cast your (foggy) mind back to the night before and try to remember if you said or did anything awful.
Most of us have woken up with a hangover at some point and struggled through the day wondering how on earth a few drinks can leave us feeling so wretched ("Can I die from a hangover?" has more than 15 million hits on Google). But the medical definition of a hangover has finally been settled by a German court, which last month ruled it should be classed as an official illness.
Hangover is a proper 'illness' now.
In February, a team of German scientists studying hangovers, writing in the American Journal of Clinical Nutrition, found hangover prevention beliefs such as "beer before wine and you'll feel fine" were scientifically unproven.
"If alcohol was invented today – and treated like a new food additive coming to the market – the recommended safe dose would be about a glass of wine per year. We're very harsh on new food and drink meeting certain criteria, but we have a blind spot towards alcohol because it's so embedded into our culture.
"There's dehydration," says Prof Nutt. "There's inflammation of the brain, which is on a par with a bad cold or flu. The pumping headache is caused by an increase in blood pressure. In fact, incidences of strokes go up on a Sunday and Monday due to weekend alcohol consumption.
"And then there's hangxiety - hang-over anxiety - which is due to something called Gaba (gamma-aminobutyric acid)," explains Prof Nutt. "Alcohol targets the Gaba receptor, which sends messages through the brain and nervous system to inhibit the activity of nerve cells, which calms the brain. Alcohol stimulates Gaba, which is why you begin to unwind and feel happy when you drink."
There's dehydration. There's inflammation of the brain. The pumping headache is caused by an increase in blood pressure.
After the first few drinks, you start blocking glutamate, which causes anxiety, and this leads to the devil-may-care stage that sees you buying another round and missing your last train. However, the body registers these imbalances and begins to bring Gaba levels down and glutamate back up. So, overnight, the happy, carefree you in the pub becomes the anxious, mildly depressed one you see in the bathroom mirror the next morning.
"Then there's sleep," continues Prof Nutt. "After four hours of going to bed, withdrawal kicks in, so you don't sleep particularly well. Water before bed to stave off a hangover, or drinking a lot of beer, means getting up early to go to the lavatory, which affects sleep further. And a lack of sleep makes hangxiety worse."
Glutamate also plays a role in memory, and after around seven drinks the glutamate system is blocked. And if you can't remember what you said in the pub, it further increases hangxiety.
After four hours of going to bed, withdrawal kicks in, so you don't sleep particularly well. And a lack of sleep makes hangxiety worse.
Professor David Nutt, Imperial College London
So do hangovers get worse with age? "There's no evidence they do," says Sally Adams, an assistant professor in health psychology at the University of Bath. "Liver mass reduces with age, so your liver is less effective at metabolising alcohol.