The retinohypothalamic tract is a pathway in the human body that plays a crucial role in regulating the circadian rhythm, or the body's natural overnight sleep-wake cycle. There are a few synonyms used interchangeably to refer to this pathway, including the optic-hypothalamic tract, retinohypothalamic pathway, and the photic pathway. In basic terms, this pathway is responsible for transmitting information from the retina in the eye to the hypothalamus in the brain, where it impacts the production of the hormone melatonin. Different terms may be used depending on the specific context, but all refer to this key aspect of human biology.