When you eat refined sugar, which means a very sweet (high calorie) food with no nutrients or fiber, the sugar gets absorbed into your blood stream very fast - unnaturally fast. This makes your blood sugar become unnaturally high. This unnaturally high blood sugar makes your body dump a great deal of insulin into your blood stream to counteract the sugar. This rapidly drops your blood sugar to an unnaturally low level.
It's easier to see the effects of this in other people, especially children. If you give a child something very sweet to eat on an empty stomach, very soon the child displays a great deal of energy, but probably not very good judgement. Often the child's behavior becomes a problem (for the adult, not for the child). If the child is able to burn off the energy in physical play, the child's behavior may not be a problem.
But then, when the child's blood sugar rapidly drops, the child displays fatigue, irritability, and perhaps some whining or crying. Then the child's behavior is a problem for both the adult and the child.
The physical effects are the same for an adult. After eating refined sugar, you may feel that you have lots of energy because your blood sugar has risen suddenly. This may feel good to you, but your focus and concentration may be poor. Then your blood sugar will drop equally suddenly. At that point you will feel tired, irritable, and mentally foggy. Often, this is the point where you decide that you need more refined sugar. Then your blood sugar goes up the roller coaster again, only to drop down just as fast.
The fact is you can never be satisfied when you are eating sugar. When you eat sugar, 45 minutes to an hour later (depending on your individual metabolism) you will want to eat sugar again, then 45 minutes later, you will want it again, etc. On and on, up and down, gaining more weight and feeling more emotionally unstable and tired as you go.