Understanding a restless child

Why is it that children are sometimes restless and at other times are concentrated? Is this because of bodily changes or are there other explanations?
I thought I might look for an answer using Google and found there is a relationship between insulin and restlessness in diabetes, which suggests that restlessness and eating are interconnected, because insulin regulates the intake of sugar in the cells of our bodies. So I searched further including the word "schedule" and got advice over how eating small meals over the day and not eating before sleeping is healthy. The blog with the most common sense I quickly found is here. Making the meal an enjoyable family happening is important but it is easier said than done... Try eating with someone that you're mad at or when you're angry at something. It's not that easy to leave problems behind at the table. In my experience other people can help a lot to change eating habits just by being there and bringing their positive thinking into the equation.
But then, maybe my three-years-old was just being jealous when I tried to teach a neighbour woman some French...