ghrelin
Ghrelin is essentially the body's “meal alarm.” As your stomach empties between meals, it releases ghrelin, which travels to the brain and shouts “time to eat.” Levels climb in the hour before a habitual mealtime and drop sharply once you have eaten, making it almost the mirror image of the satiety signals.
Ghrelin is a peptide hormone produced mainly by specialized cells in the empty stomach. To become active it must be modified by an enzyme that attaches a fatty-acid tail (acylation). Acyl-ghrelin acts on receptors in the hypothalamus, where it stimulates the appetite-driving AgRP neurons, and it is the only known circulating hormone that reliably increases hunger. Beyond appetite it also stimulates growth hormone release, influences reward pathways, and affects glucose metabolism.
Because ghrelin promotes eating, it has been an obvious drug target, but blocking it has proven disappointing for weight loss — the appetite system has too much redundancy. Interestingly, ghrelin behaves unexpectedly after some bariatric surgeries: removing much of the stomach lowers ghrelin and may help explain reduced hunger after sleeve gastrectomy.
After a night of poor sleep, ghrelin tends to rise and leptin to fall, helping explain why sleep deprivation increases appetite and snacking.
Sleep loss tips the ghrelin–leptin balance toward hunger.
Ghrelin and leptin are often described as opposing players, but they act on overlapping hypothalamic circuits rather than being simple on/off switches.