Other side effects include irritability, insomnia, fatigue, and nausea, and skin dryness. Actually there has been very little research on the effectiveness of light therapy. Some even suggest it is just a placebo effect. It can't hurt too much though and some find it very positive. The biological cause of depression seems to be caused, at least in part, by lower amounts of neurotransmission.
Neurotransmitters are chemicals in the brain that send signals.
The melatonin hypothesis claims that people with symptoms of SAD have less melatonin because of a disruption in its release. Melatonin is released only at night, usually at the beginning of the night and helps to induce sleep. In people with SAD, melatonin is released later during the night and therefore they get less into their system. This doesn't mean though that you should run out and get melatonin tablets from the health food store. There is very little research on the effectiveness of melatonin supplements.