Background: There is evidence of seasonality in bipolar affective disorder (BAD) and the preponderance of atypical symptoms in bipolar depressive episodes is also seen in winter type Seasonal Affective Disorder. Differences in seasonal symptoms between BAD and appropriate comparison populations have been scrutinised only in small studies. Methods: Symptoms described on the Seasonal Pattern Assessment Questionnaire (SPAQ) were compared between 183 patients with BAD and 468 patients consulting their general practitioners. Statistical analyses were adjusted for differing age and gender distributions between the two groups. Results: Compared with the general practice patients, subjects with BAD reported greater seasonal fluctuations in mood (p = 0.003). On one measure BAD subjects reported increased seasonal changes in social activity (p<0.001) and greater weight fluctuation over the year (p = 0.001). The most striking differences were in sleep patterns; BAD subjects slept significantly more throughout the year, and slept for a mean of 1.8 h more in winter than in summer (versus a 1.0 h difference in the general practice group, p<0.001). Against 20% of the general practice group, 46% of BAD patients rated seasonal changes in well-being to be at least a moderate problem. Limitations: The SPAQ was designed as a screening instrument for Seasonal Affective Disorder, not for studies of this nature. Some of the reported differences, notably in social activity and weight changes, may reflect secondary psychosocial effects of BAD. Conclusions: Seasonal changes, most notably winter hypersomnia, should be identified in patients with BAD. These symptoms may respond to treatments such as light therapy that are used in recurrent winter depression. (C) 2011 Elsevier B.V. All rights reserved.