Ghrelin

Ghrelin is produced mainly by the stomach and rises before expected meals, falling after eating. It acts on the hypothalamus to increase appetite and food-seeking, and is the only well-established hormone known to do so from the gut.

It matters because it rises during weight loss and stays elevated well after a diet ends — measured in some studies a year or more later. Combined with the fall in leptin, that produces a persistent hunger drive that makes maintaining weight loss genuinely harder than achieving it, for physiological rather than moral reasons.

In practice ghrelin also rises with short sleep, which pairs with falling leptin to make sleep-deprived eating a measurably different experience. It is partly habitual too: meal timing entrains its pattern, which is why hunger arrives at your usual mealtimes.