I forgot to add some info about the way the insulin level affects you:

Say you eat a big choccy bar and down a can of coke: Your blood sugar level rapidly rises and your body wrongly releases too much insulin. After a while the very high level of insulin overcomes the blood suagr level and you effectively crash. Your bodies next response is to buffer your energy level with adrenalin which is usually the 'fight or flight' response reaction and you get all the effects of that adrenalin - usually irritability, feeling down etc.