All patients must meet the diagnostic criteria for recurrent major depression or bipolar mood disorder. Seasonal affective disorder (SAD) is then a sub-type specifier used to describe temporal variations of these disorders. As such, SAD is not considered a stand-alone diagnosis or comorbid condition to recurrent major depression or bipolar disorder. Common presentations include the initiation or worsening of depressive symptoms during the autumn or winter months, and full remission during the spring or summer months, or hypo-manic or manic symptoms presenting during spring or summer months.
Circadian and neurotransmitter factors are likely to contribute to the pathophysiology of SAD, although the exact mechanism of action remains ill-understood. The suprachiasmatic nucleus (SCN) of the hypothalamus is being increasingly recognised as the 'master regulator' of several systems implicated in seasonal mood regulation. Diminished light during the autumn and winter may cause a phase shift in various circadian rhythms, including sleep-wake cycle, body temperature, hormone levels, and melatonin secretion.
The agening of the population is the major force driving the epidemic of chronic diseases. In 2000, 7% of the Chinese population were aged 65 years or older, and more than 400 million Chinese adults are now aged 20–39 years. If current trends continue, by 2040 the group aged 65 years and older will have increased to almost 20% of the population.The ageing of the population alone is predicted to produce a 200% increase in deaths from cardiovascular diseases in China between the years 2000 and 2040.