There are two types of neurons within a particular area of your hypothalamus that are relevant here. There are the so-called POMC neurons (proopiomelanocortin neurons) that tend to act as more of a break on appetite by way of another hormone called melanocyte-stimulating hormone ... When you're getting a lot of sunlight and you're viewing a lot of sunlight, that system is ramped up. This is why appetite is lower in the summer months than it is in the winter months. This is true in animals, and this is true in humans.