This presentation reviews the mechanisms whereby the prevailing light:dark cycle and the circadian clock, i.e., the suprachiasmatic nucleus (SCN), impact the synthesis in and secretion of melatonin from the mammalian pineal gland. Regardless of their specific activity pattern, i.e., nocturnality versus diurnality, melatonin is exclusively produced at night and the duration of the rise is proportional to the length of the daily dark period. The extension of the daily light period with artificial light defers the nighttime melatonin increase and, likewise, exposure to manufactured light in the morning before sunrise truncates nocturnal melatonin production. The suppressive actions of light are mediated by a small percentage (1- 2%) of the retinal ganglion cells [intrinsically photosensitive retinal ganglion cells (ipRGC)], which contain a specialized photopigment (melanopsin), that is responsive especially to blue light wavelengths (460–480 nm) and mediates the inhibitory actions of light on nocturnal melatonin production; this is referred to as circadian visual system. A melanopic spectral efficiency function (melanopic lux) details the stimulation of melanopsin-containing ipRGC by polychromatic light. Axons of ipRGC project to the SCN in the retinohypothalamic tract and, via a partially defined molecular network, regulate circadian processes and prevent the clock from stimulating pineal melatonin production. The neural pathway from the SCN to the pineal gland involves cells in both the central and peripheral sympathetic nervous system. In conclusion, light at night disturbs the circadian clock and the endogenous melatonin rhythm; these changes significantly impact the physiology of the organism.