Prevalence of seasonal affective disorder and seasonality of mood and behaviour in Iceland - Methodological implications for revisiting the latitude hypothesis.

BACKGROUND

Seasonal Affective Disorder (SAD) is thought to be more prevalent at higher latitudes, yet early research in Iceland reported a surprisingly low prevalence. Prior studies were based on markedly different methodological approaches. We provide an update on prevalence of SAD and seasonal fluctuations in Iceland and propose that distinction between these two conditions is crucial to clarify the relation between prevalence and latitude.

METHODS

We randomly selected a representative sample of 15,000 Icelandic citizens from the population registry. Among them, 9319 could be contacted and invited to complete the Seasonal Pattern Assessment Questionnaire (SPAQ). Participants with high Global Seasonality Score (GSS ≥ 11) were invited to complete a semi-structured clinical interview via phone to obtain a clinical diagnosis of SAD.

RESULTS

Among 1384 participants (response rate = 14.85 %) the weighted prevalence of elevated seasonality was 17.86 %. Seasonality was more pronounced among younger individuals, women, students, unemployed individuals, single participants, and those without children. It correlated moderately with depressive symptoms as assessed with the Patient Health Questionnaire. Of the high scorers, 111 completed the clinical interview, among which 18 met diagnostic criteria for SAD. This resulted in a weighted prevalence of 2.24 %.

LIMITATIONS: Response rates were rather low. Only a subset of participants with elevated seasonality completed the clinical interview.

CONCLUSIONS

SAD remains relatively rare in Iceland, but seasonal mood and behaviour fluctuations are as common as in other areas at high latitude, underscoring the need to distinguish between subjective seasonality and clinical SAD.