The suprachiasmatic nucleus (SCN) is located in the hypothalamus, specifically:
In the anterior hypothalamus
Immediately superior (above) the optic chiasm
On both sides of the third ventricle (paired nuclei)
Its name literally reflects this position:
“Supra-” = above
“Chiasmatic” = optic chiasm
Functionally, the SCN serves as the body’s master circadian clock, regulating sleep–wake cycles and other circadian rhythms in response to light signals received from the retina via the retinohypothalamic tract.