ILLUSTRATION: STEPHANIE CARTER/GETTY IMAGES
THE HORMONE MELATONIN plays a major role in our sleeping and waking cycles, and its production is controlled by exposure to light. This means that, as it starts to get dark in the evening, or we are exposed to less light, melatonin secretion rises to assist us in feeling tired, eventually helping us to fall asleep. Conversely, in the morning when we wake up, exposure to light shuts down melatonin production, so we can stop feeling sleepy and get out of bed.