How does daylight set the brain's clock, and why does the clock sit on top of the optic chiasm? · Grey Matter

The master circadian clock is the suprachiasmatic nucleus, a small pair of nuclei in the front of the hypothalamus resting just above the optic chiasm, as its name says.

How does daylight set the brain's clock, and why does the clock sit on top of the optic chiasm?

The master circadian clock is the suprachiasmatic nucleus, a small pair of nuclei in the front of the hypothalamus resting just above the optic chiasm, as its name says. Its neurons keep a rhythm of about 24 hours on their own, and it is reset each day by light through the retinohypothalamic tract, fibres that leave the optic nerves near the chiasm and come from a special class of light-sensitive retinal ganglion cells. The clock then times sleep, body temperature, cortisol and melatonin across the day, which is why light in the evening shifts sleep later.