Background: Multiple lines of evidence suggest that the onset and course of bipolar disorder is influenced by environmental light condi-tions. Increased suppression of melatonin by light (supersensitivity) in patients with bipolar disorder has been postulated as an endo-phenotype by several studies. However, due to methodological shortcomings, the results of these studies remain inconclusive. This study investigated melatonin suppression in euthymic patients with bipolar I disorder using evening blue light specifically targeting the melanopsin system. Methods: Melatonin suppression was assessed in euthymic patients with bipolar I disorder and healthy controls by exposure to monochromatic blue light (λ_max = 475 nm; photon density = 1.6 × 10¹³ photons/cm²/s) for 30 minutes at 2300 h, administered via a ganzfeld dome for highly uniform light exposure. Serum melatonin concentrations were determined from serial blood sampling via radioimmunoassay. All participants received mydriatic eye drops and were genotyped for the PER3 VNTR polymorphism to avoid or adjust for potential confounding. As secondary outcomes, serum melatonin concentrations during dark conditions and after monochromatic red light exposure (λ_max = 624 nm; photon density = 1.6 × 10¹³ photons/cm²/s) were also investigated. Changes in subjective alertness were investigated for all 3 lighting conditions. Results: A total of 90 participants (57 controls, 33 bipolar I disorder) completed the study. Melatonin suppression by monochromatic blue light did not differ between groups (F_1,80 = 0.56; p = 0.46). Moreover, there were no differences in melatonin suppression by monochromatic red light (F_1,82 = 1.80; p = 0.18) or differences in melatonin concentrations during dark conditions (F_1,74 = 1.16; p = 0.29). Healthy controls displayed a stronger increase in subjective alertness during exposure to blue light than patients with bipolar I disorder (t₈₅ = 2.28; p = 0.027). Limitations: Large interindividual differences in melatonin kinetics may have masked a true difference. Conclusion: Despite using a large cohort and highly controlled laboratory conditions, we found no differences in melatonin suppression between euthymic patients with bipolar I disorder and healthy controls. These findings do not support the notion that supersensitivity is a valid endophenotype in bipolar I disorder.