The pineal gland in the brain produces the hormone melatonin, which induces sleepiness. When and how much melatonin is released is controlled by your exposure to sunlight. When the sun sets, melatonin production increases and you get tired. When the sun rises, melatonin production stops, so you can wake up and start your day. The blue light emitted by smartphones, computers, TVs and even lightbulbs messes with this carefully calibrated cycle—which is why watching TV or texting at night can leave you lying wide-eyed in bed.