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The most important difference between Seasonal Affective Disorder (SAD) and other types of depression is the atypical way in which it manifests itself. Both manic depression and major depressive disorder would have been evolutionarily disadvantageous for our ancestors, while SAD would have been beneficial in numerous ways. Today however, that manifestation of SAD has become an evolutionary throwback in our modern society and remains a problem for millions worldwide.

Seasonal Affective Disorder and its more popular sub category, Sub-Seasonal Affective Disorder, strikes annually from late autumn to early spring (and lasting the duration of the winter). Overall, some of the symptoms of SAD and S-SAD occur throughout the general population, but in varying degrees (Murray, 2003). Characterized by lethargy, hypersomnia, weight gain, craving carbohydrates and foods with a high fat content, SAD almost completely impairs functioning for those diagnosed (Smith, 2005). To be diagnosed with SAD, one must have recurring SAD symptoms for at least two concurrent years and not have those symptoms be related to seasonal stressors (or psychosocial factors) (Rawana, 2006). Sub-Seasonal Affective Disorder (S-SAD) is distinguished from SAD by the lesser severity of its symptoms. It is not correct to suggest that SAD and S-SAD are caused simply by the weather or the "holiday blues" because they extend for a period of months rather than the few weeks around the holidays (Partonen, 2001). Also, holidays are a relatively recent invention and have no real evolutionary value. Both SAD and S-SAD in modern society, although they were evolutionarily advantageous for the proto-human, have proved to be simply a regression to our more primitive history.
Without the aid of artificial lighting, the proto-human was confined to the hours of sunlight provided (photoperiod), and was completely unable to turn on a bedside lamp or a kitchen light to continue working. Because of this, the natural photoperiod in which he lived was considerably shorter during the winter months and consequently lengthened in the summer. The same is true today, yet we have the artificial lighting to aid and extend our hours of wakefulness and sleep. Photoperiod is of extraordinary value to the etiology of SAD according to the Phase Shift Hypothesis. The Phase Shift Hypothesis theorizes that the hours of sunlight during the day influence our circadian rhythm (which is responsible for hormone and neurotransmitter regulation,

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