Light alerts the biological clock to slow down the secretion of which hormone that promotes sleep

Light alerts the biological clock to slow down the secretion of melatonin hormone that promotes sleep. Melatonin hormone is a hormone that is produced by the pineal gland in animals and regulates sleep and wakefulness. . The hormone is used to regulate night and day cycles or sleep-wake cycles, darkness causes the body to produce more melatonin, which signals the body to prepare for sleep. Light decreases melatonin production and signals the body to prepare being awake. 

To kill bacteria from our body especially from stomach