Leptin

Leptin is the hormone fat tissue releases to tell the brain the body has enough fuel. The brain reads it as the signal to settle hunger and let appetite ease.

In the lean context, leptin signaling is what makes a calorie deficit livable. When the signal is honest, hunger arrives at meals and eating less feels manageable. When the signal is suppressed, the brain reads the body as starving even when the day's calories are correct, and the urge to eat overrides the plan.

Two failure modes. The acute one: a few short nights drop leptin, spike ghrelin (the hunger hormone), and push the next day's intake about 300 calories higher without registering. The chronic one: months of unbroken deficit suppress leptin out of proportion to the fat lost, the brain compensates with rising hunger and falling activity, and adherence collapses. The fix is feeding the signal, not powering through it.

What raises it

• Sleeping. Leptin runs on a daily rhythm with a peak during deep sleep. A full night restores the signal that short sleep had suppressed.
• Sun exposure. Morning daylight sets the body clock that times leptin release across the day. Days that start indoors blunt the rhythm and the appetite that runs on it.

What lowers it

• Short sleep. A few short nights drop leptin and flip the satiety signal. The next day the brain reads the body as hungrier than it is, and total intake climbs about 300 calories without registering.
• Alcohol. Drinking fragments sleep architecture, which blunts the deep-sleep window where leptin recovers. The morning after drinking is a low-leptin morning.