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. This topic was selected for two reasons. Pineal gland plays a key role in controlling circadian and seasonal rhythmic processes in virtually all vertebrate species. Also, the avian pineal gland is an excellent model for studying the mechanism of the circadian processes in general, since this organ is relatively simple in structure and it possesses all the known features of a fully functioning circadian "biological clock".