Circadian rhythm is your body's 24-hour internal clock governing sleep, alertness, hormone release, body temperature, and metabolism. When aligned with your natural rhythm (and your chronotype), you wake refreshed, perform at peak during high-stakes tasks, and sleep deeply. When misaligned (late bedtimes, irregular sleep, wrong work timing), you operate in perpetual fatigue. You feel "fine" because you've normalized exhaustion. One month of proper sleep and you realize you've been running on fumes.