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Circadian rhythms are a feature of almost all living cells. When isolated from external stimuli, organisms exhibit self-sustaining cycles in behaviour, physiology and metabolism, with a period of approximately 24 hours. Sunlight is used to entrain this endogenously generated rhythmicity to the earth’s rotation to provide a three-dimensional perception of the world plus the fourth external dimension, time.

I hypothetically trace the evolution of the system from blue-light photoreception by the earliest marine organisms to the rise of photosynthetic bacteria creating an aerobic environment. Species now had to contend with the exogenous threat of UV radiation plus the endogenous toxic by-products of oxidative phosphorylation. Mammalian systems incorporated tools from earlier systems but refined them to the present highly integrated system of control of physiology. Cryptochrome, the blue light photoreceptor, is incorporated but photoreception now the domain of opsins in the retina. The system having central control by the suprachiasmatic nucleus within the hypothalamus, well placed to receive light information from the retina but also communicating with other brain areas and the periphery through neural and hormonal links.

This system still has relevance to us as humans within our modern environment, since a de-synchronised circadian system can contribute to a number of diseases.