Ghrelin

The hormone ghrelin is the opposite of leptin. It's the hunger hormone that tells your hypothalamus that your stomach is empty and that you need to eat. Its major purpose is to make people eat more.

Ghrelin levels are normally highest before a meal and lowest afterward. Obese people, oddly, have low ghrelin levels but are more sensitive to its effects, according to studies. Overeating may be a result of this sensitivity. One explanation for the difficulty in losing weight is that calorie restriction generally causes an increase in ghrelin levels, keeping you hungry. Additionally, metabolism slows down, and leptin levels drop. So it's a good idea to listen to your body and eat when you're hungry because ghrelin levels are highest before a meal.