It is well known that the pineal hormone melatonin is suppressed by light. The melatonin suppression by dim light has also been suggested as a possible trait marker of bipolar disorder. However, there is large inter individual dierences in the light responses. It is possible that methodological factors may contribute to the variable responses observed. Most studies in the past have been conducted in dierent seasons under dierent lighting conditions. It is possible the external lighting conditions may aect the melatonin suppression to dim light. Hence we examined the melatonin suppression by dim light in (1) a large group of subjects randomly tested once in one of the four seasons ( part one) and ( 2) small group of subjects tested in all seasons ( part two). Subjects were placed in a dark room from 2100 h to 0230 h. Light exposure was between midnight and 0100 h in a sitting position. Blood samples were collected at regular intervals for measurement of plasma melatonin. No statistically signiยฎcant seasonal dierences in the percentage suppression of melatonin were found in both parts of the study ( p 4 0 . 5). The study suggests that seasonal changes in external lighting do not aect the melatonin suppression by dim light in healthy controls.