Think about the word "breakfast." All night long (save those rare occasions when you raid the fridge at 3 a.m. to pick at leftovers), you go without food. This fasting lasts from dinner until breakfast and causes your metabolism to go into hibernation and slow down.
There are several biological reasons you would want your metabolism to slow down while you're spending some quality time with your pillow. It helps you to relax and fall asleep, and it allows your body to devote the majority of its energy to reparative work while you're sleeping. This is why eating right before bed is not a great idea. It forces your body to expend energy digesting this new food rather than storing it up for when you have to do the daily household chores.
The most important thing to remember, however, is that your metabolism will not "turn on" and move back up to full speed until it knows you are awake. When you break the fast by eating your favorite morning meal—or even just a piece of whole wheat toast or fruit—you're letting your body know to get that metabolism up and running. To jump-start your metabolism and burn the most calories over the course of the day, eat as soon as you get up.
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