The factors that regulate your appetite are not as straightforward as was once believed. The interactions among ghrelin, leptin, insulin, glucose, thyroid hormones, growth hormone and other determinants of hunger or satisfaction are intricate and only partially understood. To confuse matters further, sleep-inducing hormones, such as melatonin, exert their own influences on your appetite, and changes in sleep patterns can alter the way your brain responds to hunger signal.
During a nap, the appetite centers in your brain are stimulated by falling glucose levels and counter-regulatory hormones, making you feel hungry again. Thus, the more insulin your pancreas produces in response to a given meal, the greater the subsequent rebound in your appetite.