Which raises the question: how does the suprachiasmatic know the correct external time—that is, if it is day or night? In chronobiological lingo, the suprachiasmatic nucleus has to be entrained by external cues, the zeitgebers (time givers). Sunlight is, of course, the most important zeitgeber. The location of the suprachiasmatic nucleus at the intersection of the left and right optic nerves is not coincidental; this location makes it ideally situated to receive raw data about whether it is light or dark outside the skull.
