Intrinsically Photosensitive Retinal Ganglion Cells (ipRGCs) refer to specialized cells in the human retina that are sensitive to light and influence the body's circadian rhythm. These cells contain the protein melanopsin which allows them to respond directly to light, unlike other retinal cells that rely on signals from rods and cones. Synonyms for ipRGCs include melanopsin-containing retinal ganglion cells (mRGCs) and photosensitive retinal ganglion cells (pRGCs). These cells play a key role in regulating our sleep-wake cycle, reaction to light, and overall health. Understanding the role of ipRGCs in the body can help researchers develop therapies for sleep disorders and other conditions influenced by circadian rhythms.