Digest and metabolize. This would mean that early in the day, you digest food and put it to work. Your body temperature is higher, you fidget more, your body repairs some of the damage you did when you worked out last night.

Later in the day, the food you digest sits around. Your body temperature is lower, you fidget less, your body performs less maintenance and repair. But the body isn't wasteful; it's still built to survive periodic famine. So, those extra calories that are just sitting around get stored away for later.