The neuroendocrine loop model revisited: Is it valid or even relevant?

Biological rhythms and the biological clocks that control them are fundamental properties of most living organisms, ranging from several forms of bacteria to multicellular plants and animals. These properties share many formal and biochemical properties, and affect all aspects of physiological function, ranging from control of transcription to metabolism to cell cycle to behavior. Because these rhythms are expressed similarly in single-celled and multi-cellular organisms, and because clock function pervades all levels of biological organization, many authors have suggested that biological clocks are properties of all cells.¹