Many factors play a role in preparing your body to fall asleep and wake up. You have an internal “body clock” that controls when you’re awake and when your body is ready for sleep. This clock typically follows a 24-hour repeating rhythm (called the circadian rhythm). The rhythm affects every cell, tissue, and organ in your body and how they work. This clock is in sync with certain cues in the environment. Light, darkness, and other cues help determine when you feel awake and when you feel drowsy. For example, light signals received through your eyes tell a special area in your brain that it is daytime. This area of your brain helps align your body clock with periods of the day and night.