A number of studies have shown that the central oscillator in the SCNs is not the only biological clock in the human body, and that the genes of the human biological clock are also expressed cyclically in many peripheral tissues, such as the liver, heart, skin, and lymphocytes.
These peripheral clocks are co-ordinated by the central clock in the SCNs, which is synchronized with the day/night cycle by the direct influence of the ambient light. This central clock then communicates with the peripheral ones via the nervous and circulatory systems, so that these peripheral clocks can in turn synchronize themselves with the central oscillator. These peripheral clocks may also be influenced by other entraining factors, such as the timing of meals.

In addition to affecting the production of melatonin indirectly, via the pineal gland, the suprachiasmatic nuclei may also release certain peptides directly into the cerebrospinal fluid, such as arginine vasopressin (AVP) during the daytime and vasoactive intestinal peptide (VIP) at night.

Lastly, both SCNs also send signals, though indirectly, to another very important structure: the pineal gland. In birds, reptiles, and fish, this small gland located at the top of the brain is sensitive to light and co-ordinates some cyclical phenomena on its own. In mammals, however, though the pineal gland does retain its ability to produce secretions cyclically (specifically, the hormone melatonin, at night), it does not constitute a clock on its own; instead, its cyclical synthesis of melatonin is controlled by timing signals that it receives from the SCNs.

Each day, the pineal gland begins to produce melatonin (sometimes called the “sleep hormone”) as night falls. As the level of melatonin in the blood rises, body temperature falls slightly, and the individual feels sleepier and sleepier. The melatonin level remains high for just about 12 hours, then starts falling again in the early morning, as daylight inhibits this gland’s activity.