Serotonin is a hormone that is also classified as a neurotransmitter, which sends, or transmits, nerve impulses to the brain. It is found in the brain and the digestive tract, and can even be found in some fruits and vegetables. Serotonin is necessary for the regulation of many functions within the body, and when you do not have enough of it you can develop serotonin deficiency syndrome, which brings with it several symptoms.
Serotonin plays a major role in mood disorders, especially depression. According to the Mayo Clinic, when the balance of neurotransmitters, specifically serotonin, is altered, it will affect mood. The lower the level of serotonin in the brain, the greater the risk is for developing a depressive disorder. This is the principle that anti-depressants are based on. Some anti-depressants are called selective serotonin reuptake inhibitors (SSRIs), and they work by blocking the ability of the brain's nerve cells to reabsorb (reuptake) serotonin. This leaves more serotonin available in the brain to work at sending nerve impulses, thereby reducing feelings of depression and improving overall mood.
The level of serotonin in the brain can also affect how well you sleep. According to an article about a study in the June 6, 2006, edition of "Current Biology," scientists use the common fruit fly (Drosophila melanogaster), as a tool in sleep research, due to the fact that it has a very simple nervous system. In studies with the fly and serotonin, researchers at the University of Pennsylvania found that when levels of serotonin were increased, the flies experienced longer and deeper sleep. This coincides with what scientists have known for years, which is that serotonin has an effect upon sleep cycles. As noted above, low serotonin levels can also lead to depression, which can also lead to disrupted sleep.
Serotonin also plays a role in food cravings. A paper published Nov. 6, 2005, by the Hypoglycemic Health Association of Australia explains that part of serotonin's job as a neurotransmitter is to relay the sense of satisfaction when it comes to food intake and appetite. When the regulation of appetite becomes unbalanced, it can lead to eating disorders, such as binge eating, which can be a precursor to obesity. The HHAA states that sugar will initiate the production of serotonin in the body, so a body low in serotonin will automatically crave sweets and simple carbohydrates, in order to produce enough of it. Unfortunately, this more often leads to sugar addiction than it does to adequate serotonin production. The National Institutes of Health report that serotonin not only affects binge eating, but low levels can also be related to bulemia and anorexia nervosa.
Serotonin is produced in the digestive tract, and a report from Vitamin Research Products explains that if there is a deficiency of serotonin, the digestive system is vulnerable to irritable bowel syndrome, constipation and digestive upset. Serotonin is also a muscle stimulant that helps regulate the movement of the stomach muscles, which explains why it helps prevent constipation.