Dehydration occurs when your body loses more fluid than you've taken in. When you get dehydrated, this means the water in your body has dropped below the level needed for normal body function. Small changes in water levels often go unnoticed and don't cause problems, but losing larger amounts can make you feel sick.
Many people think that dehydration only occurs when doing strenuous exercise or spending a lot of time outdoors on hot day – this isn't true. It's easy, especially if you're drinking a lot of coffee or soda, to forget to drink hydrating drinks such as water or sports drinks. This can cause you to feel tired, dizzy and generally unwell.