When light strikes the retina, it signals the suprachiasmatic nucleus to alter melatonin production by the pineal gland.
The full process works as follows:

1. Photoreceptors in the retina detect light and send signals along the retinohypothalamic tract to the suprachiasmatic nucleus (SCN) in the hypothalamus.
2. The SCN acts as the body’s master circadian pacemaker, regulating the circadian rhythms of various physiological processes, including sleep-wake cycles.
3. In response to light signals from the retina, the SCN inhibits the production and release of melatonin by the pineal gland.
4. Melatonin is a hormone produced by the pineal gland, mainly at night in darkness. It plays a key role in regulating the body’s circadian rhythms and sleep-wake cycle.
5. During the day when light hits the retina, melatonin production is suppressed. At night in darkness, melatonin levels rise, promoting sleepiness.