Evening melatonin and bright light administration induce additive phase shifts in dim light melatonin onset


Address reprint request to Anna Wirz-Justice, Centre for Chronobiology, Psychiatric University Clinic, Wilhelm Klein Strasse 27, CH-4025 Basel, Switzerland.
E-mail: anna.wirz-justice@unibas.ch


Abstract:  In healthy young men, administration of a single light pulse (5000 lux for 3 hr) or a single melatonin pill (5 mg) at 20:40 hr under controlled constant routine conditions of <10 lux, yielded a phase delay and a phase advance, respectively, in the circadian marker of dim light melatonin onset 24 hr later. Phase shifts after combining the two interventions were additive. Melatonin suppression is not necessary for a phase shift by light, and melatonin is not a ‘weak’ Zeitgeber relative to bright light when ambient lighting is strictly controlled.