Junk foods typically contain a lot of sugar. Sugar is a source of quick energy. This may increase your energy level for a short period of time, particularly if you have not eaten anything in awhile. Of course eating a regular, healthy meal would do the same thing and would likely give you energy for a longer period of time. Junk foods are generally low in nutrients and high in fat. Thus in the long term, junk foods are very poor providers of energy for your body and can actually decrease your energy level and ability to concentrate.