…wakes you up and prevents you from sleeping properlyYes and no. It's a fact that coffee contains caffeine, which can have a stimulating effect. But everyone reacts differently to coffee, and you have to take other factors into account before confirming or refuting the claim. Drinking a cup in the evening may well lead to sleep problems for someone who's not used to coffee. But regular drinkers find the effects of caffeine get less the more they drink. Older people may even find a cup helps them get to sleep. This is because the problems they have sleeping are often caused by lack of circulation to the brain. Coffee helps address this because it starts stimulating the neurotransmitters around a quarter of an hour after you drink a cup.
…is bad for your heartIt depends on the quantity. To this day experts are divided on the question of whether coffee is harmful for the heart or not. There have been various studies showing that people who drink between three and five cups a day suffer a heart attack or stroke less often than those who drink none at all or substantially more than five cups. For people with certain pre-existing conditions (such as high blood pressure), however, coffee is something to be treated with caution, because in such cases it really can be harmful. Those affected should consult their doctor.
…dehydrates youFalse. The claim that coffee dries you out isn't true. While it's true that the chlorogenic acid it contains acts as a diuretic, this doesn't remove fluid from the body. On the contrary: coffee consists almost entirely of water, and according to the Swiss Federal Office of Public Health can even have a hydrating effect – even though water should of course be the main source of hydration. Drinking coffee also boosts the loss of sodium from the body, which lowers the percentage of extracellular water without fluid being removed from the cells.
…irritates the stomach
True. The chlorogenic acid mentioned above can irritate the lining of the stomach, which means that some people get heartburn after drinking coffee. This is most likely to happen on an empty stomach, because the coffee can cause a reflux of stomach acid into the oesophagus. But the problem's easily solved by adding a little milk to your coffee or drinking it with a meal. If you have a sensitive stomach it's also a good idea to choose highly roasted coffees such as Arabica. The more intensively the beans are roasted, the less chlorogenic acid they contain.