Vitamin B-12 enhances the phase-response of circadian melatonin rhythm to a single bright light exposure in humans

Eight young males were subjected to a single blind cross-over test to see the effects of vitamin B-12 (methylcobalamin; VB12) on the phase-response of the circadian melatonin rhythm to a single bright light exposure. VB12 (0.5 mg/day) or vehicle was injected intravenously at 1230 h for 11 days, which was followed by oral administration (2 mg x 3/day) for 7 days. A serial blood sampling was performed under dim light condition (less than 200 lx) and plasma melatonin rhythm was determined before and after a single bright light exposure (2500 lx for 3 h) at 0700 h. The melatonin rhythm before the light exposure showed a smaller amplitude in the VB12 trial than in the placebo. The light exposure phase advanced the melatonin rhythm significantly in the VB12 trail, but not in the placebo. These findings indicate that VB12 enhances the light induced phase-shift in the human circadian rhythm. Copyright (C) 1996 Elsevier Science Ireland Ltd.