Approximately 10% of all cases of mood disorders show a seasonal pattern of recurrence but this figure is thought to be higher in bipolar disorder with some studies showing that 20% of this group have some element of seasonality in their symptoms. Some studies have shown peaks of depression in the autumn and peaks of mania in early spring and late summer while other studies have shown little difference in hospitalization rates with the seasons. A ten-year Spanish study of 325 people with bipolar disorder found that 25.5% of them showed a seasonal pattern of symptoms. There were no demographic differences between those with seasonal and non-seasonal forms of bipolar disorder. People with seasonal bipolar disorder were more likely to have the bipolar II form of the condition and were more likely to have more depression than mania. However, people with a seasonal aspect to their bipolar disorder were no more likely to be suicidal, hospitalized or suffer from psychosis.
Goikolea, J. M. ... [et al] - Clinical and prognostic implications of seasonal pattern in bipolar disorder : a 10-year follow-up of 302 patients Psychological Medicine November 2007 37(11), 1595-1599