<p>Melatonin is the principal hormone of the pineal gland and can be described as the body's signal for the dark phases. It affects people by promoting sleep, and has an activating effect on nocturnal animals. Melatonin in the pineal gland is continuously produced from serotonin and stored within, and only released in the dark. Light suppresses the melatonin secretion. If no melatonin is released, the melatonin level in the blood drops by degradation processes. Since melatonin is the most important outward measurable marker for the circadian phase of the internal clock, the effect of the melatonin suppression is generally equated with the circadian effect, i.e. the biological effects of light on the circadian clock.</p>