Our bodies run on an in-built 24-hour clock embedded in a part of the brain called the suprachiasmatic nucleus
(SCN). Its 20,000 nerve cells sit in the middle of the brain above the back of the eyes and on top of a structure called the hypothalamus. These are the body’s master timekeepers, setting the rhythm for sleeping, waking, eating, and hormone release.
Even in a test tube, cells from the SCN keep time. They are stufed with molecules called transcription factors, which change the production levels of other molecules on a 24-hour cycle. The master regulators are known as BMAL and CLOCK. Together, these two molecules activate the production of molecules called periods and cryptochromes. As levels of periods and cryptochromes rise, they feed back to BMAL and CLOCK, switching production of again. This causes the amount of these molecules to go up and down in cycles, forming the basis for a precise timekeeper.
Like any clock, the SCN can run fast or slow, so the time is reset, or entrained, every day by daylight. This is done by light-sensitive cells in the back of the eye known as intrinsically photosensitive retinal ganglion cells. They don’t
produce images when they detect light: instead they send signals to the SCN via a bundle of nerve tissue called the retinohypothalamic tract, syncing the master clock, which in turn messages the rest of the body about the time.