When I was a teenager, I once fasted for three days in a row, not because I was trying to lose weight but because I was curious what would happen. I discovered that hunger was not a constantly increasing phenomenon, but came and went in more or less predictable waves of roughly 20 minutes in length, which I later connected to blood sugar drops driven by conditioned insulin releases.

See, just like the salivary glands in Pavlov's dogs, your pancreas is a gland which has nerves running into it. It can be conditioned to release insulin under specific circumstances. A really common conditioned stimulus is the clock showing meal time.