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.