---
title: "Ghrelin — AP Psychology Definition & Exam Guide"
description: "Ghrelin is the stomach's 'hunger hormone' that signals you to eat. Learn how it pairs with leptin, links to sleep loss, and shows up on the AP Psych exam."
canonical: "https://fiveable.me/ap-psych-revised/key-terms/ghrelin"
type: "key-term"
subject: "AP Psychology"
unit: "Unit 7"
---

# Ghrelin — AP Psychology Definition & Exam Guide

## Definition

Ghrelin is a hormone produced by the stomach that stimulates hunger, signaling the brain that it's time to eat. On the AP Psych exam, it's the 'hunger hormone' that works opposite leptin to regulate the biological drive to eat.

## What It Is

Ghrelin is a [hormone](/ap-psych-revised/key-terms/hormone "fv-autolink") your stomach releases when it's empty. It travels to your [brain](/ap-psych-revised/unit-1/2-overview-of-the-nervous-system/study-guide/4EFLv8T9ARX14r9M "fv-autolink") and basically says "hey, time to eat," which is why it gets nicknamed the **hunger hormone**. As ghrelin levels rise, your appetite goes up; after you eat and your stomach fills, ghrelin drops.

In AP Psych, ghrelin lives in the endocrine system ([Topic 2.2](/ap-psych-revised/unit-2/2-thinking-problem-solving-judgments-and-decision-making/study-guide/gHqqU9CdMYyy4lPn "fv-autolink")), the body's slower chemical-messenger system that uses hormones instead of neurotransmitters. It also anchors the biological side of motivation (Topic 7.2). The big idea: hunger isn't just "I feel like eating." It's partly driven by hormones doing math in the background. Ghrelin pushes you toward food, and its partner hormone leptin pushes you away from it. The two work like a thermostat keeping your eating in balance.

## Why It Matters

Ghrelin sits in [Unit 2](/ap-psych-revised/unit-2 "fv-autolink") under Topic 2.2 (The Endocrine System) and carries over into Unit 7's Topic 7.2 on motivation. It's a concrete example of how biology drives behavior, which connects to the broad theme that thinking and behavior have physical, chemical roots. Knowing ghrelin lets you explain hunger as a biological process, not just a feeling, and that's exactly the kind of cause-and-effect reasoning the exam wants. It also pairs naturally with [drive-reduction theory](/ap-psych-revised/key-terms/drive-reduction-theory "fv-autolink"): ghrelin creates the internal need state (hunger) that drives you to act (eat) to restore balance.

## Connections

### [Leptin (Unit 2)](/ap-psych-revised/key-terms/leptin)

[Leptin](/ap-psych-revised/key-terms/leptin "fv-autolink") is ghrelin's mirror image. Ghrelin says "eat," leptin says "stop, you're full." Think of them as the gas pedal and the brake for hunger. If you remember one, you've basically learned the other by contrast.

### Circadian Rhythm and Sleep Loss (Unit 2)

Skip [sleep](/ap-psych-revised/unit-1/5-sleep/study-guide/qpkX61RMlTEfmjbA "fv-autolink") and your hormone balance shifts. Ghrelin goes up and leptin goes down, so you feel hungrier than you should. This is why sleep, eating, and weight questions often show up together on the exam.

### [Drive-Reduction Theory (Unit 7)](/ap-psych-revised/key-terms/drive-reduction-theory)

Ghrelin is the biological trigger behind a [hunger](/ap-psych-revised/unit-4/6-motivation/study-guide/ejKphjzI71jpngsa "fv-autolink") drive. The hormone creates an internal need state, you eat to reduce it, and balance is restored. It's drive-reduction theory shown in actual body chemistry.

### Obesity and Eating Disorders (Unit 7)

When ghrelin and leptin signals get disrupted, eating behavior can go off the rails. This links the hunger hormones to topics like obesity and anorexia nervosa, where the body's normal appetite regulation breaks down.

## On the AP Exam

Ghrelin shows up most often in multiple-choice questions that test one clean fact: which hormone stimulates hunger? Expect stems like "Which hormone tells your body it's hungry?" where ghrelin is the answer and leptin is the trap. A common variation asks which hormone does NOT signal hunger, where leptin would be the correct pick. The 2023 SAQ Q1 used this term, so you may need to explain or apply it in a short written response, not just recognize it. When you write about ghrelin, define it as the stomach hormone that increases hunger, then connect it to its opposite (leptin) or to a theory (drive-reduction) to earn full credit. You may also see it bundled with sleep questions, where lack of sleep raises ghrelin.

## Ghrelin vs Leptin

These two get swapped constantly because they both regulate hunger. Ghrelin comes from the stomach and INCREASES hunger. Leptin comes from fat cells and DECREASES hunger (tells you you're full). Memory trick: Ghrelin = Grumbling stomach (eat). Leptin = Less hunger (stop).

## Key Takeaways

- Ghrelin is a stomach hormone that stimulates hunger, nicknamed the 'hunger hormone.'
- Ghrelin rises when your stomach is empty and falls after you eat.
- Ghrelin and leptin work as opposites: ghrelin says eat, leptin says stop.
- Lack of sleep raises ghrelin and lowers leptin, increasing appetite.
- Ghrelin lives in Unit 2 (endocrine system) and Unit 7 (motivation), and it illustrates drive-reduction theory in action.
- On MCQs, ghrelin is the hormone that signals hunger; don't confuse it with leptin.

## FAQs

### What is ghrelin in AP Psychology?

Ghrelin is a hormone made by the stomach that stimulates hunger and signals your brain that it's time to eat. In AP Psych it appears in the endocrine system (Topic 2.2) and motivation (Topic 7.2) as the 'hunger hormone.'

### Is ghrelin the hormone that makes you hungry or full?

Hungry. Ghrelin INCREASES hunger; it's leptin that signals fullness. If a question asks which hormone stimulates hunger, the answer is ghrelin.

### How is ghrelin different from leptin?

Ghrelin comes from the stomach and turns hunger ON, while leptin comes from fat cells and turns hunger OFF (signals you're full). They work as opposites to keep eating in balance.

### Does lack of sleep affect ghrelin levels?

Yes. Sleep loss tends to raise ghrelin and lower leptin, which makes you feel hungrier. That's why sleep, hormones, and appetite often appear together on the exam.

### Is ghrelin on the AP Psychology exam?

Yes. It commonly appears in multiple-choice questions about which hormone stimulates hunger, and it was referenced in the 2023 SAQ Q1, so you should be able to define and apply it.

## Related Study Guides

- [7.2 Specific Topics in Motivation](/ap-psych-revised/unit-7/specific-topics-motivation/study-guide/Btf2NwFm4LH5lV3swOLs)

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