Ghrelin

Ghrelin is the hunger hormone, released mainly by the stomach when it is empty and falling after a meal. It signals the brain that energy is needed, and it is one of the few hormones known to increase appetite rather than suppress it.

It rises before habitual meal times, which is why hunger arrives on a schedule you have taught it rather than purely on physiological need. Change your eating pattern for a week or two and the pattern of hunger shifts with it, which is a genuinely useful thing to know when starting a new routine.

It also has roles beyond appetite, including stimulating growth hormone release, which is where its name comes from, and influencing reward pathways so that food becomes more attractive rather than merely more necessary.

Dieting raises it, and it stays raised. Studies following people after weight loss find ghrelin remaining elevated for a long time, which is part of the physiology behind why maintenance is harder than the loss and why the difficulty is not a character failure.

Short sleep raises it too, alongside lowering leptin, which is one plausible route by which poor sleep makes appetite harder to manage.

Protein and fibre suppress it more effectively than refined carbohydrate does, which is one mechanism behind the satiety advantage of those foods, and one reason meal composition affects how a diet feels rather than only what it totals.

In practice, expect hunger to rise as a diet progresses, build meals around protein and fibre, protect sleep, and plan maintenance periods rather than relying on willpower.