Seasonality and circadian phase delay: prospective evidence that winter lowering of mood is associated with a shift towards Eveningness.

BACKGROUND: Lewy's phase shift hypothesis (PSH) of seasonal affective disorder (SAD) asserts that winter depression is caused by an abnormal delay of circadian phase in winter. The aim of this study was to further investigate the PSH by testing the novel hypothesis that winter pattern seasonality of mood in the general population is associated with a phase delay of circadian rhythms in winter.
METHODS: Using a random community sample of 244 adults in Melbourne, seasonality of mood was measured prospectively as the within-subject difference between self-reports of behavioural engagement in summer and winter across 3 years. The Horne-Ostberg Morningness-Eveningness questionnaire (MEQ) was used as a self-report estimate of circadian phase. Phase delay in winter was measured as the within-subject difference between summer and winter estimates of circadian phase, with relative shifts towards Eveningness being interpreted as phase delay.
RESULTS: As expected, a significant association was found between winter pattern seasonality of mood and within-subject phase delay in winter (r=0.17, P<0.01). LIMITATIONS: Circadian phase was operationalised on the MEQ. While a number of studies have shown the MEQ to correlate with physiological measures of circadian phase, the findings await replication using a more direct measure of the circadian system.
CONCLUSIONS: A positive association was found between lowered mood in winter and winter phase delay amongst a random community sample. This provisional finding adds support to Lewy's PSH by demonstrating that phase delay may be causally important not just in clinical cases of SAD, but across the continuum of mood seasonality.