Light therapy treatment is most commonly used for treating different kinds of depression. Recently, light therapy has been used in treating Parkinson’s disease, not just the depression of Parkinson’s patients but to help alleviate motor problems as well.
Bright light stimulates two neurotransmitters that Parkinson’s sufferers lack: serotonin and dopamine. When light reaches a certain level of intensity, it inhibits melatonin which in turn limits the production of dopamine. By limiting the melatonin, in theory, you will have better production and use of dopamine in the brain.
Melatonin, a sleep-related hormone, has also been associated with seasonal affective disorder. Melatonin is made with the amino acid called tryptophan. This hormone has been linked to depression and is produced at higher levels of darkness.  Darkness also increases the production of melatonin. In most cases, this is a good thing. As daylight wanes, most people are preparing to sleep for the night. Darkness helps melatonin to aid in this natural process. In the winter when the days are shorter and darker, melatonin is produced which can then lead to symptoms of seasonal affective disorder.
Serotonin is a neurotransmitter, derived from tryptophan. In order for messages to be transmitted properly, neurotransmitters must pass a sort of baton between these runners. If serotonin levels are low, certain messages cannot be sent correctly or quickly enough through the nervous system. As darkness aids in producing melatonin, sunlight aids in the production of the neurotransmitter serotonin. Without sufficient sunlight, the body may not produce enough serotonin. This can cause a drop in energy levels, a general sense of confusion or “fogginess” and other symptoms typically associated with depression.
Recent case studies of light therapy for treating Parkinson’s Disease patients involving bright light therapy treatment have had positive results with marked improvement in bradykinesia and rigidity in most patients while being exposed for only ninety minutes. After two weeks the study also showed the light therapy Parkinson’s treatment increased mood, improved sleep, decreased seborrhea, reduced impotence and increased appetite. This change was so significant; some patients were able to have their medication reduced by 50% without the loss of efficiency.