Patients with bipolar disorder suffer from sleep disturbances, even in euthymic phases. Changes in sleep are frequent signs of a new episode of (hypo)mania or depression. Cognitive behavioral therapy for insomnia is an effective treatment for primary insomnia, but has not been introduced to patients with bipolar disorder. The aim is to compare cognitive behavioral therapy added to 'treatment as usual' with just 'treatment as usual'. The investigators hypothesize that cognitive behavioral therapy will improve quality of sleep, stabilize minor mood variations and prevent new mood episodes in euthymic patients with bipolar disorder and insomnia.

Inclusion Criteria:

• Fulfilling criteria for SCID-1-verified bipolar I or II disorder
• Euthymic, as defined by Montgomery Åsberg Depression Rating Scale (MADRS) not higher than eleven, and Young Mania Rating scale (YMRS) not higher than five.
• Fulfilling DSM-IV criteria for primary insomnia or insomnia related to another mental disorder, as assessed by the Insomnia Interview Schedule (IIS).

Exclusion Criteria:

• Being or having been in a defined affective episode the last month before inclusion
• Hospitalization in the last two months before inclusion
• Working night shifts
• Sleep apnea
• Medical conditions incompatible with participation.
• Inability to cooperate in the 3-week initial phase before randomization.