What governs our circadian rhythm ?

Light / dark: The brain takes in light in decreasing and increasing amounts depending on time of day and time of year, and a center in the brain, the suprachiasmatic nucleus, placed over the optic nerves, takes the information, and sets the "internal clock". This nucleus tells the pineal gland when to secrete melatonin, the hormone that tells us when we're supposed to get sleepy.