Circadian rhythm

Circadian rhythm is the body's internal clock, a roughly 24-hour cycle that times most of the systems involved in being lean. Hunger, sleep, cortisol release, and energy across the day all run on this clock. The clock is set by daylight on the eyes.

In the lean context, a clock that fires on time means hunger arrives at meals and sleep arrives at night. A clock that has drifted (from late screens, no morning daylight, irregular sleep times) means hunger spikes at midnight and the day starts low. The hormones that drive ghrelin, leptin, cortisol, and melatonin all run on this same clock.

The first hour after waking is where the day gets set. Bright light hitting the eyes within that window starts the clock cleanly. Days that start indoors push the rhythm later, and the rest of the day follows.

What sets it

• Sun exposure. Morning daylight is the strongest signal the clock has.
• Walking. A morning walk delivers daylight while you cover daily steps. One habit pays into the clock and the deficit at the same time.
• Sleeping. A consistent bedtime and wake time give the clock a rhythm it can plan around. Hormones, hunger, energy, and mood all run smoother when the schedule is regular.