Hunger without eating comes from a hormone called ghrelin, which your stomach releases on a schedule built around your normal meal times, whether or not food is actually available.
Ghrelin travels to the hypothalamus, the part of the brain that controls appetite, and triggers the physical sensations of hunger such as stomach growling and food focused thoughts.
This means your body can feel hungry at 1 p.m. simply because you usually eat lunch then, even if you are not truly short on energy.
Ghrelin levels naturally rise and fall in waves throughout the day, so a wave of hunger that feels intense at first often fades within 15 to 20 minutes if nothing is eaten.