Although many dieters avoid carbs, they play an important role in promoting a good mood and delivering energy. Carbohydrates are the preferred fuel source for the body, and they raise levels of serotonin, a brain chemical that improves and stabilizes the mood. However, not all types of carbs are mood-boosters. Sweets tend to cause rapid spikes - and then drops - in blood sugar, leading to tiredness and bad moods. Whole grain carbs such as whole-wheat bread, brown rice, and cereal are absorbed more slowly into the bloodstream, promoting stable blood sugar and energy levels.