Leptin

Leptin is secreted by fat cells roughly in proportion to fat mass, and acts on the hypothalamus to signal that energy stores are adequate. It suppresses appetite and permits energy-expensive functions such as reproduction and thyroid activity to proceed normally.

It matters because leptin falls quickly during energy restriction — faster than fat mass does — which raises hunger, lowers spontaneous movement and reduces thyroid output. That is a substantial part of why long diets get harder rather than easier, and why weight regain is so common.

In practice, leptin deficiency is vanishingly rare and the discovery of the hormone did not produce the obesity treatment it promised: people with obesity generally have high leptin and reduced sensitivity to it, so giving more does nothing. Sleep deprivation lowers leptin and raises ghrelin, which is one clear mechanistic route from poor sleep to overeating.