English: Visible light entering the eye and the cascading positive and negative signalling pathways to neuronal structures in the mamallian brain that may follow: When the eyes are exposed to sunlight, the pineal gland's melatonin production is suppressed, resulting in the secretion of hormones that promote wakefulness. Conversely, in the absence of light, the pineal gland synthesizes melatonin unabated, leading to feelings of drowsiness and facilitating the onset of sleep.