The organ that is responsible for circadian rhythm is the Suprachiasmatic Nucleus (SCN).
The Suprachiasmatic Nucleus (SCN), which is responsible for circadian rhythm is located in the hypothalamus of your brain, which is the master clock which regulates your body's circadian rhythms, and receives light signals from your eyes to synchronize the body's internal clocks, which include controlling of the production and release of melatonin by the pineal gland for sleep.
The way the Suprachiasmatic Nucleus (SCN) works is specialized cells in the retina detect light and then send signals via the retinohypothalamic tract directly to your Suprachiasmatic Nucleus (SCN).
The Suprachiasmatic Nucleus (SCN) also processes these light cues, and acts as the central pacemaker which also coordinates billions of other cellular blocks that are throughout your body.
And based on the light-dark cycle, the Suprachiasmatic Nucleus (SCN) signals your pineal gland to release melatonin, which is a sleep inducing hormone at night and suppress the melatonin during the day.
The Suprachiasmatic Nucleus (SCN) also sends signals to other brain areas and also peripheral issues like your heart, liver and kidneys to keep their clocks also in sync with the master clock and the environment.
The key structures involved in the circadian rhythm are the Suprachiasmatic Nucleus (SCN), which is the brain's main and primary circadian pacemaker.
The hypothalamus, which is your brain's region where the Suprachiasmatic Nucleus (SCN) resides.
The retina, which detects light and is the primary cue.
And the pineal gland that produces melatonin under Suprachiasmatic Nucleus (SCN) control.