Vasodilation is the process of widening of the blood vessels. As the arterioles are dilated, the blood flow is increase due to the increase of resistance and consequently, the blood pressure is decreased. Dilation happens when the body is in need of oxygen to replenish the areas where it is being consumed. Demands for oxygen happen when we are currently doing an exercise or working.
The opposite of which is the vasoconstriction where it narrows the arteries and the arterioles where the flow of blood is being restricted.
When we eat, we need more oxygen in order for the cells to utilize nutrition. We also need oxygen in order to maintain or metabolic activities. However, during sleeping we need less of it compared when we are doing something.