Light Therapy part 1

Overview of Light Therapy

Seasonal Affective Disorder & Latitude

Many studies in the United States have reported a significant effect of latitude on prevalence, with an increase in prevalence with increasing latitude. Rosen et al (1990) working in Alaska, found a SAD rate of 8.9% and a subsyndromal SAD rate of 24.9%. Mersch et al (1999) Investigated the relationship between the prevalence of seasonal affective disorder and latitude and found the mean prevalence of SAD to be two times higher in North America compared to Europe.
A significant positive correlation was found between prevalence and latitude in North America. For Europe there was a trend in the same direction. Rosen et al (1990) surveyed three geographic areas (New Hampshire, New York, and Florida) and resurveyed one (Maryland) to compare symptomatic seasonal changes in mood and behaviour at four different latitudes. Rates of winter SAD and SSAD were found to be significantly higher at the more northern latitudes. A population survey of seasonality in six representative cities in Japan revealed significant regional differences in seasonal variations of mood, length of sleep, and weight. The proportion of individuals reporting high seasonality in the two northernmost cities was significantly higher than that in the other areas (Okawa et al, 1996). Magnusson (2000) also found that SAD was more prevalent at higher northern latitudes, but that the prevalence varied across ethnic groups. In the southern hemisphere, work in New Zealand and Australia has shown a similar effect of latitude. It is assumed that the incidence of SAD increases with increasing latitude up to a point, but does not continue increasing all the way to the poles. There seems to be interplay between an individual's innate vulnerability and degree of light exposure (Mersch et al, 1999). For example some individuals who work long hours inside office buildings with few windows may experience some symptoms all year round. Some very sensitive individuals may note changes in mood during long stretches of cloudy weather.

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