(A) Blood glucose, (B) body weight and (C) serum melatonin level in experimental animals. Diabetes was induced by a single intraperitoneal dose of streptozotocin (STZ, 100 mg/kg). Animals were subsequently exposed to a light/dark cycle of 6/18 h for 3 weeks (C6/18 and DM6/18 groups) or to a regular 12/12 h light/dark cycle (C12/12 and DM12/12 groups). Glucose was measured in blood samples obtained from the tail vein weekly within 3 weeks after STZ administration. Melatonin was determined in serum samples obtained from blood 3 weeks after STZ administration following behavioural testing. The significance of the difference was estimated by an analysis of variance (ANOVA) with Dunnett’s post hoc test (* p < 0.05 and ** p < 0.01 vs. C12/12, ## p < 0.01 vs. DM12/12). Abbreviations: DM12/12, diabetic group exposed to a 12/12 h light/dark cycle; C6/18, group exposed to a 6/18 h light/dark cycle; DM6/18, diabetic group exposed to a 6/18 h light/dark cycle.