Sugar is a form of carbohydrates, and carbohydrates are good for you as long as you eat the right kinds and in the right amounts. Avoid refined processed sugar whenever possible. Foods baked with white sugar, such as cookies and cakes, and prepackaged foods such as soda or candy are often the culprits behind a sugar rush.
The reason people reach for sugary foods is the instant energy boost they receive from simple sugars, meaning sugars that break down quickly to be converted into energy. Increased energy, alertness and giddiness have all been associated with the upswing a sugar rush. Unfortunately, the initial stages of a sugar rush do not last. The energy from simple sugars can last from 15 to 40 minutes, and once it's gone you feel even more tired than you were before you ate.
The sugary food took energy to digest, meaning you are left with less energy than before you ate. Since simple sugars give you no nutritional benefit, your body gets nothing from the food you ate. All of this causes the fatigue or sleepiness you feel after the sugar rush is over.
When you eat foods that are high in sugar, your body creates a high dose of insulin to combat the sugar high. Once the sugar rush is over, the high dose of insulin leaches additional sugar from the body, consequently causing low blood sugar. Low blood sugar or hypoglycemia can make you feel hungry even if you just ate.
Evidence from a Princeton study indicates that reactions in the brain cause you to crave more sugar once you have eaten some. In the study, rats were given a sugary drink and a piece of whole wheat bread. Within a month, the rats doubled their consumption of the drink and went without the nutrients from the bread. The professor overseeing the study concluded that humans react in much the same way. The last symptom of a sugar rush is the craving for more sugar. Sugar cravings can be just as intense as the craving for coffee or drugs, and they can induce withdrawal symptoms.