The first effect of eating junk food is its impact on energy levels. Many people skip breakfast or other meals throughout the day, choosing instead to grab a quick snack or a soft drink. The junk food causes energy levels to spike, which people like, but then energy levels will quickly plummet, sending one back into the kitchen for another quick snack. In addition, junk food can cause moodiness, and make it difficult to get enough sleep at night, so energy levels are never restored to normal.