circadian rhythm

A circadian rhythm is your body's built-in daily cycle — a roughly 24-hour pattern that quietly rises and falls inside you, telling your body when it is day and when it is night. It is the reason you naturally feel sleepy in the late evening and alert in the late morning, even if you cannot see a clock or the sky. The word comes from Latin: circa means about and dies means day, so circadian literally means about a day. Think of it as an internal timer that has been ticking since before you were born, gently nudging dozens of things in your body — your sleepiness, your body temperature, your hunger, your hormones — onto a regular daily schedule so they happen at the right time of day.

This rhythm is generated from the inside, by a tiny cluster of brain cells in the hypothalamus called the suprachiasmatic nucleus, which acts as the body's master clock. Left completely alone in a cave with no light or clocks, your rhythm would still run, but it tends to drift a little off 24 hours, so each morning it is reset by signals from the outside world — most powerfully by daylight reaching your eyes. Daylight tells the master clock the real time, the clock keeps every cell and organ in step, and your whole body stays synchronized to the turning of the planet.

Why it matters: when your inner clock and your daily life line up, you sleep well, feel energetic by day, and your body runs smoothly. When they clash — after flying across time zones, working night shifts, or staring at bright screens deep into the night — you get jet lag, poor sleep, foggy thinking, and, over the long run, a higher risk of health problems. Honoring your circadian rhythm, by getting bright light in the morning and dimming it at night, is one of the simplest ways to keep both body and mind in good shape.