When your body breaks down food for energy, it converts carbohydrates into glycogen. Some is stored in your liver and drip feeds your body and vital organs with energy. It only has 4 hours worth of supply. Also, the body requires a certain amount of calories to stay alive. If we drop beneath this, the body starts a chain reaction of biochemical and physiological adaptations to ensure it survives. One of them is to slow down your metabolism. Once your body feels starvation your metabolic rate drops. This can be as much as 20% within 24 hours. Add to this the breakdown of protein in the body and you have a sluggish, acidic environment which isn't conducive to providing you with much energy. The longer this cycle persists, the weaker your immune system will be.
We need to top up energy levels with some protein, fruit, nuts and seeds in between main meals. In simpler times, we would forage for berries and seeds. Think like a caveman (or caveperson if this manages to get to Harriet Harman). The danger time is 2-5. This is most likely an unproductive time of the day for you. Also, have you ever suffered the wrath of a hungry child? Imagine a hungry employee who isn't nice at the best of times. Don't wait for it to happen, keep some healthy snacks with you.