Serotonin is an aromatic chemical which acts as a neurotransmitter in the body. It has a similar effect to Prozac which makes us feel relaxed and calm. In the body serotonin is broken down through oxidative deamination, catalysed by monoamine oxidase, followed by oxidation to 5-hydoxyindoleacetic acid, by aldeyde dehydrogenase. 

It has many effects on the body such as:

As a result one can feel nauseas, and mood is effected. If there is a decrease in serotonin levels in the brain this leads to craving for starchy and sweet foods like chocolate. Once the serotonin levels rise again then one feels good.