Radioprotective effect of endogenous melatonin secretion associated with the circadian rhythm in irradiated rats.

We investigated the radioprotective effect of endogenous melatonin release at different times associated with the circadian rhythm on head and neck radiotherapy. Two groups of animals were subjected daily to 8 Gy single fraction radiotherapy in the head and neck region from 5:00 to 6:00 (the morning group) or from 19:00 to 20:00 (the evening group). Corresponding untreated groups served as controls.