Seasonal affective disorder: some epidemiological findings from a tropical climate

To determine whether a predominantly summer-focussed pattern of seasonal affective disorder (SAD) exists in tropical northern Australia.

A mail survey containing a modified form of the Seasonal Pattern Assessment Questionnaire (SPAQ) was administered to 176 households in the city of Townsville (latitude: 19 degrees south).

Using previously reported screening criteria, 9% of respondents reported a degree of summer impairment indicative of SAD, while the incidence rate for winter SAD was 1.7%. This summer-winter ratio is typically reversed in more temperate latitudes.

Excessive heat and humidity were reported to be the two most influential environmental factors affecting mood and behaviour. It is likely that respondents meeting criteria for summer SAD represent the extreme end of a spectrum of summer-related mood and behaviour change that affects many individuals in northern Australia. Strategies for further clinical and epidemiological research on SAD in tropical climates are proposed.