The retinohypothalamic tract is a photic input pathway involved in the circadian rhythms of mammals.^[1] The origin of the retinohypothalamic tract is the intrinsically photosensitive Retinal Ganglion Cells (ipRGC), which contain the photopigment melanopsin. The axons of the ipRGCs belonging to the retinohypothalamic tract project directly to the suprachiasmatic nuclei via the optic nerve and the optic chiasm.^[2][3] The suprachiasmatic nuclei receive and interpret information on environmental light, dark and day length, important in the entrainment of the 'body clock'. They can coordinate periferal 'clocks' and direct the pineal gland to secrete the hormone melatonin.

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