A circadian rhythm is an approximately 24 hour periodicity in the various biochemical and physiological processes of living beings. The pacemaker for generating circadian rhythm in mammals is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Circadian oscillations are synchronized with environmental light cycles via several photoreception systems. The neurons of the SCN receive light input signals from the retina by way of the retinohypothalamic tract. The synchronization of circadian rhythm with the environmental light cycles involves transcription of several clock genes in the SCN...