Seasonal affective disorder (SAD) was originally described as a syndrome in which depression developed during autumn or winter and remitted the following spring or summer.1,,2 Since then two subtypes of SAD have been defined in the scientific literature: winter SAD and summer SAD, the former being by far the more frequent.2,,3 Most winter SAD patients have ‘atypical’ depressive symptoms such as increased sleep duration, increased appetite and weight, and carbohydrate craving. Patients with winter SAD may experience a reversal of their winter symptoms in summer: mild hypomania, elevated mood, increased libido, social activity and energy, and decreased sleep requirements, appetite and weight. Seasonality, the tendency to experience seasonal changes in mood and behaviour, can be viewed as a dimension, ranging from those who show no seasonal changes to those who show more extreme changes with the seasons. Seasonal changes of mood and behaviour are common throughout the population.4–,6 A survey in the Washington area found that approximately 4% of the population have winter SAD and over 10% more have sub‐syndromal winter SAD.4 Some 27% of respondents reported that changes with the seasons were a problem for them. Another survey, in New York City, indicated approximately 6% with potential clinical severity, 18% reporting milder symptoms that were bothersome, and 35% noting symptoms but without complaint.6 Thus, many people experience mild analogues of winter SAD. A considerable number of studies by researchers around the world have demonstrated that light therapy (daily scheduled exposure to artificial light) is an effective treatment for winter SAD.1,6–,10 An internal pacemaker that matches internal rhythms to the 24‐h day drives circadian rhythms.9–,13 In humans, the master circadian pacemaker is the bilaterally paired suprachiasmatic nuclei (SCN) in the anterior hypothalamus. …