summary
A 40-year-old woman with insulin-dependent diabetes mellitus was treated successfully with phototherapy for a seasonal affective disorder. Following sessions of phototherapy she developed hypoglycaemias and required less insulin. A review of the literature showed that melatonin has an inhibiting effect on insulin sensitivity. The melatonin secretion, which is suppressed by phototherapy, may cause an immediate decrease in the plasma glucose levels. This decrease may well be important for patients with insulin-resistant diabetes mellitus and seasonal affective disorder.