Passage 1 Passage 2 Passage 3 Based on Passage 1, why do people feel tired at the end of each day? The Sleep-Wake Cycle Their brains have not had enough healthy food. Their brains are trying to start a new rhythm From the time people wake up to the time they sleep and then wake again, their bodies are following the workings of an inner clock. There is a reason for this pattern, which is called a circadian rhythm. It has to do with how light affects the human brain. As the sun sets, the lack of natural light coming through the eyes causes the brain to release a chemical called melatonin. Their brains are receiving too many signals. Their brains are telling them it is time to sleep. 1 2 3