Ghrelin

Ghrelin is the hunger hormone. The stomach releases it when empty, the brain reads it as the urge to eat, and a meal brings it back down. The cycle is a normal part of how hunger runs.

In the lean context, the calorie deficit is easier when ghrelin behaves and harder when it doesn't. A deficit with steady ghrelin feels like normal hunger that arrives at meals. A deficit with disrupted ghrelin feels like reactive snacking that won't quit.

Sleep, body clock timing, and meal composition are the levers that keep ghrelin clean. Short sleep and irregular meal timing both push ghrelin higher than it should be. Hard cardio spikes ghrelin sharply, which is why people often eat back the calories they burned in a hard session. Walking doesn't have that effect, which is part of why it's the cleanest way to add daily activity in a deficit.

What keeps it steady

• Sleeping. Short sleep spikes ghrelin and drops leptin (the satiety hormone) the next day. A few short nights flips both, and total intake climbs about 300 calories without registering.
• Sun exposure. Morning daylight sets the body clock that times ghrelin release across the day. Days that start indoors push hunger later.
• Walking. Walking adds daily activity without the ghrelin spike that hard cardio creates.