This is a disease characterised by:
It occurs seasonally, with affected individuals suffering during the darker winter months. Most patients (80%) are women in their late twenties, although this finding may in fact be due to increased reporting of symptoms in this group rather than other groups not suffering from the disorder.
Some studies have shown alterations in the melatonin blood level rhythms in affected people during the darker winter months. It has been hypothesised that changes in the melatonin profile brought about by the longer dark days of winter may cause SAD. Studies on sufferers in the Arctic suggested that SAD sufferers had abnormally high daytime levels of melatonin (when melatonin should in act be low in the day). Melatonin, as previously described, acts to promote sleepiness, perhaps accounting for the tiredness and fatigue seen in SAD. However as little research has been done, nothing can be said with any degree of certainty here.
Bright light therapy (or phototherapy), where patients are exposed to bright light through the eyes for as little has half an hour a day does seem to have a beneficial effect. The mechanism for how this works is not yet known. It may indeed work be helping to reset the circadian rhythm of the body, possibly involving melatonin. It may also work by altering levels of the brain neurotransmitter serotonin. Low levels of serotonin have been associated with depressive illness for some time. Anti-depressant drugs such as Prozac act to increase brain serotonin levels. Exposure to light appears to raise serotonin levels. Perhaps this is why phototherapy benefits SAD patients.
Clearly this is an area in need of great research. We must wait for more data before ascribing too much strength to the melatonin hypothesis of SAD. Only after this will we have the information we need to adequately deal with the disorder.