Where the master clock sits and how light reaches it
The suprachiasmatic nucleus (SCN) is a small paired structure in the hypothalamus, just above the optic chiasm, containing on the order of thousands of neurons. It functions as the body's central circadian pacemaker, generating a rhythm close to but not exactly 24 hours.
Light is the dominant cue that keeps it aligned. Specialized retinal cells containing the pigment melanopsin signal ambient light through the retinohypothalamic tract directly to the SCN, which is why morning light is so effective at resetting timing.
How one clock coordinates the whole body
The SCN does not act alone. It broadcasts timing signals, through neural pathways and hormones such as melatonin from the pineal gland, to peripheral clocks present in nearly every tissue, including liver, muscle, and gut.
When these peripheral clocks fall out of sync with the SCN, as in shift work or jet lag, sleep, metabolism, and alertness can be disrupted. This is general educational information, not medical advice.