What causes hangovers?

Dehydration
Alcohol is a diuretic, ie a drug that increases urination and flushes fluids from the body. Drinking coffee the next morning only increases this problem as coffee is also a diuretic.

Mild poisons in the drink
Complex organic molecules such as methanol and acetone are found in some drinks and are said to be responsible for hangovers rather than ethanol(alcohol). This view is by researcher Dr Ian Calder of the London based National Hospital for Neurosurgey.

Which Alcohol is worse?
Brandy is worst for hangovers then in descending order are: red wine, rum, whisky, white wine, gin and vodka. The British Medical Journal did tests that showed drinking bourbon whiskey is twice as likely to cause a hangover than the same amount of vodka.

Too much alcohol
Too much alcohol depletes the body of necessary substances required to stay healthy, these include blood sugar, vitamins and minerals.

Wine - a bad harvest
If you are drinking wine that comes from a country where a small change in the climate can make a big difference to the quality of wine, eg France, Germany, New Zealand, then in a bad season the wine contains many more substances that cause hangovers.

Wine - drinking it too young
Almost all red wines and chardonnay are matured in oak barrels so that they will keep and improve. If you drink this wine younger than three years there will be a higher level of nasties that can cause hangovers. If left to mature these nasties change to neutral substances and don't cause hangovers. As a rule of thumb wine stored in oak barrels for six months should be acceptable to drink within the first year. If the wine is stored for twelve months or more in oak barrels it should then be left for at least four years. Some winemakers have been known to add oak chips directly into the wine to enhance flavours, especially in a bad season, and this can take years to become neutral.