Illustration of the mechanism by which melatonin (red arrows) is secreted by the pineal gland (red sphere). Melatonin is a hormone that affects the modulation of sleep patterns in both seasonal and circadian rhythms. Its release is inhibited by bright light falling on the retina of the eye. However, when decreased levels of light are detected a signal is sent to the pineal gland via the suprachiasmatic nucleus (green) and paraventricular nucleus (orange) of the hypothalamus to begin secreting melatonin. For this image with English labels see C049/3476.