Humans have nine kinds of photoreceptors, but only four, OPN1 (blue, green and red cone photoreceptors) and OPN2 (rod photoreceptor) are involved in sight. five photoreceptors are non-visual, including OPN4 (melanopsin) which controls circadian rhythm and OPN5 (neuropsin) which is important for myopia prevention. In this article, we explain the importance of the non-visual photoreceptors in the eye.