The role of serotonin in the body has been studied since 1948, when serotonin was isolated from animal blood serum and its high biological activity was demonstrated. Subsequently, serotonin was detected practically in all animal species (including man), and in many plants. Although serotonin has been the subject of numerous studies, its exact role in the body remains unclear. However, it is thought that Serotonin is a key neurotransmitter in the regulation of diverse functions, including mood, sleep cycles, aggression, body temperature, and appetite.
It is thought that depleted levels of serotonin may cause a host of problems, including depression, anxiety, insomnia, difficulty concentrating, fatigue, low concentration, low self-esteem, compulsive disorders, and cravings for high-glycemic foods. This is what has increasingly become known as the Serotonin Deficiency Syndrome.
While it is quite possible that the body is simply not making enough Serotonin, or that genetic deficiencies are inhibiting serotonin uptake, a wide range of lifestyle factors have also been attributed to SDS. These can include including not getting enough sleep, high levels of stress, low exposure to sunlight, unbalanced diets, prescription medications (e.g. analgesics) as well as excessive consumption of caffeine and alcohol and the use of recreational drugs.
While the links between Serotonin, SDS and Mental illness seem to be evident, no hard evidence is available on the subject.
References and further reading:
University of Bristol (UK)
Stephen M. Stahl
No Fair! My Serotonin Level Is Low
60-Second Psych blog and podcast
When Sunlight Dwindles, So Do Serotonin Transporters
November 7, 2008 – Volume 43, Number 21, page 20