During the past decade the efficacy of light therapy and melatonin therapy for affective and sleep/wake rhythm disorders has been studied profoundly. The core problem in sleep/wake rhythm disorders is a disturbance of the circadian rhythms. Sleep/wake problems are the main complaint. For scientists these syndromes have the advantage that their cause is known. This article discusses the influence of bright light and melatonin on respectively jet lag, shift maladaptation syndrome and delayed sleep phase syndrome. Although rare, the research results are consistent and they demonstrate that the efficacy of light therapy is not based on the suppression of the melatonin production. Possible working mechanisms of bright light, respectively melatonin, are discussed. The proposed hypotheses may possibly contribute to the research concerning light therapy for affective disorders.