Your body runs on a 24-hour clock called a circadian rhythm. It helps you know when to wake up, go to sleep, and stay healthy. There’s a tiny part of your brain that acts like your body’s timekeeper. It needs blue light in the morning to stay on schedule. When your rhythm is in sync, you feel awake during the day and sleepy at night. But if it gets off track, it can make you tired, moody, or even affect your health.