Your circadian rhythm, also known as your sleep/wake cycle, is an internal clock that cycles between sleepiness and alertness at regular intervals over an approximate 24-hour period.
This cycle – comprised of a “master clock” that orchestrates many other smaller clocks – is important for regulating various processes throughout your body, such as knowing when to wake up in the morning, when to eat, and when to sleep at night. The master clock is located in the suprachiasmatic nucleus (SCN), and the primary cue is the light–dark cycle, around which most mammalian (and those of all living organisms) functions have evolved (NCBI).
The SCN is notably located in the hypothalamus within the brain, where the neurons have a dedicated link to the eye. So any time there is a change in environmental lighting, information enters the eye and is directly transferred to the master clock. The SCN then influences daily rhythms in your sleep-wake cycle, hunger-satiety, body temperature, and hormone levels.