MELATONIN (MLT) IS a main synchronizer of the sleep/wake cycle control. We performed a prospective naturalistic study that included 14 euthymic bipolar patients that suffered from insomnia (six type I, six type II and two schizoaffective disorder) according to DSM-IV-TR criteria.1 After giving their written consent, patients began the treatment with MLT. Evaluation tools used were: Oviedo Sleep Questionnaire (OSQ),2 Chinese Polarity Inventory (CPI),3 Numeric Evaluation Scale (NES),4 and Clinical Global Impressions Scale foruse in bipolar illness (CGI-BP-M).5 Side-effects and time until improvement of sleep were evaluated. Euthymia had to be confirmed by a score <30 on the CPI-Depression and <15 on the CPI-Mania. MLT was prescribed at bedtime, the dose was free, between 3-6 mgr/day and it was added to a stable regimen of medication. An analysis of the results' 'mirror-image' was performed, comparing the period of treatment with MLT, with the same period of time prior to the start of the treatment. Data analysis was performed with the statistical program spss v15.0 (spss, Chicago, IL, USA). The improvement of sleep disorders occurred in all patients, and the mean time until improvement was 3.6 weeks. The scores obtained in the OSQ were: 25.8 (prior to the treatment) versus 13.8 (after the treatment) (P = 0.001). MLT significantly improved the scores in the CPI-Depression (29.78 ± 16.25 vs 23.84 ± 13.16; P = 0.04) and in the CGIBP-M, both on the subscale for depression (2.49 ± 0.82 vs 1.88 ± 0.85; Z −2.857; P = 0.004) and for mania (1.23 ± 0.3 vs 1.05 ± 0.15; Z −2.375; P = 0.018). Improvement was also observed in the NES scores (42.43 ± 8.77 vs 44.39 ± 5.49). Visits were spaced from an average of 48.8 to 65.3 days. All patients experienced a sleep improvement in quantity and quality, and there was a reduction in the total score of the items that measured clinical depression. In one of the scales that assessed the manic pole, a reduction in the total score was observed. We know that melatonin M1 and M2 agonists, such as ramelteon or agomelatine, may also be effective for bipolar disorder and these results show that MLT can be a useful treatment as adjunctive therapy in bipolar patients with sleep alterations and helping in clinical stabilization.