Although cyclicism of some biological events was evident for thousands of years, the importance of the fourth dimension, the time in life processes have been underestimated by biologists for many years. It was only in the past few decades that we realized that life is basically a coordinated interplay between cyclic biochemical processes in widely different forms and period of times. This recognition greatly altered our understanding on how living organisms function. This book discusses one specific aspect of biological cycles: the mechanism of the circadian melatonin secretion from the chicken pineal gland.