 .The ANS is activated when we open our eyes, and the amount of light is adjusted by the iris . Non-visual impacts of light are mediated by melanopsin-based photoreceptors in the retinal photosensitive ganglia cells . Melanopsin is also found in the melanophores of the iris and inner brain. Phtosensitive retinal ganglion cells transmit light stimuli to the suprachiasmatic nucleus SCN; circadian center via the retinohypothalamic tract. In the dark, melatonin released rhythmically from the pineal gland also strongly affects SCN neuronal activity. This area receives stimulation from the visual photoreceptor system and from various areas of the brain, including the brain stem, limbic system, and cerebral cortex and contains multiple synapses with the pineal gland. The input of light into the retina, which is adjusted by the iris, is associated with the areas of the brain that regulate the ANS. Light may cause acute physiological effects such as rapid melatonin suppression, increased alertness, and increased heart rate and sympathetic nerve tonus. 