Circadian describes the natural cycle your body follows over roughly 24 hours, matching the rhythm of day and night. Your circadian rhythm controls when you feel sleepy, when you feel alert, when you get hungry, and even when your body temperature rises and falls throughout the day.

This internal clock exists in nearly all living things: plants, animals, even tiny bacteria. It evolved over millions of years as organisms adapted to Earth's rotation. Your body uses signals like sunlight and darkness to keep this rhythm synchronized with the actual day. When the sun rises, your brain releases hormones that help you wake up and feel energized. As evening approaches and darkness falls, different hormones tell your body it's time to wind down and prepare for sleep.

Scientists who study circadian rhythms discovered that even without sunlight, people's bodies naturally fall into roughly 24-hour cycles.

When you travel across time zones and experience jet lag, you're feeling what happens when your circadian rhythm gets out of sync with the local time. Your body thinks it's one time while the clock says something completely different, leaving you exhausted during the day or wide awake at midnight.