The biology behind the feeling
Lesson 11 covered how to tell hunger and appetite apart in the moment. This lesson explains what is actually happening in the background: the hormones that create the feeling of hunger in the first place, and why a deficit can make that feeling noticeably stronger over time.
Four hormones worth knowing
Ghrelin: the hunger signal
Produced mainly in the stomach, ghrelin rises before meals and falls after eating. It is often called the "hunger hormone" because it directly drives the urge to eat.
Leptin: the fullness and fat-storage signal
Released by fat tissue, leptin signals to the brain how much energy is stored. Levels track body fat fairly closely, and fall as fat mass drops during a deficit, which is one reason hunger tends to increase later into a cut.
GLP-1: released after eating
Secreted by the gut in response to food, GLP-1 slows digestion and signals fullness to the brain. It is also the hormone that modern weight-loss medications are designed to mimic.
PYY: another post-meal fullness signal
Released alongside GLP-1 after eating, particularly in response to protein, PYY works together with it to reduce appetite for the next few hours.
This is why protein-rich meals, covered back in Lesson 4, feel more satisfying than an equivalent amount of calories from fat or refined carbohydrate: protein prompts a stronger GLP-1 and PYY response.
Why a deficit gets harder to sustain over time
Lesson 18 mentioned that hunger-regulating hormones respond more strongly to a deficit as body fat drops. This is that mechanism: falling leptin, combined with a tendency for ghrelin to rise, means the biological pressure to eat genuinely increases the leaner and longer into a cut someone gets.
This is not a failure of discipline. It is a predictable, well-documented physiological response, and it is exactly why Lesson 18’s diet breaks exist: a scheduled period at maintenance gives leptin a chance to recover before continuing.
The sleep connection
Lesson 14 flagged sleep as a genuine metabolism lever without explaining the mechanism. Here it is: a landmark study by Spiegel and colleagues (2004, published in the Annals of Internal Medicine) restricted healthy young men to 4 hours of sleep for two nights and found ghrelin rose by around 28% and leptin fell by around 18%, alongside a 24% increase in self-reported hunger and a notably stronger appetite specifically for calorie-dense, carbohydrate-rich foods.
Poor sleep does not just leave you tired. It measurably shifts the hormones that drive hunger, in the direction of wanting more food, and specifically more of the less helpful kind.
Further reading
- Spiegel K, Tasali E, Penev P, Van Cauter E. (2004). Brief communication: Sleep curtailment in healthy young men is associated with decreased leptin levels, elevated ghrelin levels, and increased hunger and appetite. Annals of Internal Medicine, 141(11), 846-850.
Today’s task
Note what time you actually went to sleep last night, and compare it to what you know you need to feel properly rested. No need to change anything yet, just get an honest baseline.
Check in with yourself
- Have you noticed hunger feeling stronger on days after poor sleep?
- Which meal in your day currently gives you the strongest, longest-lasting fullness?
- Any new takeaway from today’s lesson you want to remember?
Browse high-protein recipes in the cookbook to make the most of the fullness signal protein provides.