While a certain amount of conventional lore linking sugar to nightmares exists, credible evidence of this effect is difficult to come by. If you or your children have had bad dreams after eating too much sugar, the association may seem natural to you. However, sugar itself exerts no known direct effect on dreams. Eating too much of it may affect other conditions, such as hypoglycemia, which can influence dreams.
Studies of Food and Nightmares
A 2000 article in the "Journal of the Mind and Body" cites several studies which attempted to determine the relationship between eating before bed and dreams. Several of the studies indicated that eating and overeating before bed may increase brain wave activity, and that this heightened activity could increase the vividness of dreams, including nightmares. The article places some emphasis on the effect unhealthy foods may have, but does not indicate sugar alone as a primary cause of bad dreams.
Other Chemicals in Candy and Junk Food
Other chemicals may influence your dreams. For example, nicotine patches are sometimes packaged with a warning to the effect that they may cause vivid or unusual dreams. While junk food does not contain nicotine, it may have other ingredients that have similar effects. Candy and sweets with chocolate in them will contain both theobromine and caffeine. These chemicals may disrupt sleep in sufficient quantities, and such a disruption could lead to nightmares.
External Sensations During Sleep
You may be familiar with the occasional effect external influences can have on dreams. If a telephone rings, for example, someone sleeping nearby may dream of a telephone ringing. If a clock radio is on, this may cause dreams about the music it plays. Sugary foods can conceivably have a similar influence on dreams, by causing one or more physical sensations, and a dreaming child may be vaguely aware of these feelings while sleeping. For instance, the child may experience stimulant properties from sugar. His unconscious mind may then interpret the stimulation as physical exertion or excitement within his dreams. Indigestion, which may cause esophageal pain or an upset stomach, may also lead to dreams in which the pain is felt and interpreted by the unconscious mind. Not every child experiences these physical effects from junk food, and those who do may not necessary have bad dreams, so the extent to which sugar's physical influence on the body translates to bad dreams varies from child to child.
Blood Sugar and Dreaming
According to MayoClinic.com, a child's blood sugar level may influence her dreams. When her blood sugar becomes low during sleep, a condition known as diabetic hypoglycemia can occur. Some of the signs and symptoms of diabetic hypoglycemia include nightmares, perspiration and fatigue upon waking up. Other aspects of diabetes such as the so-called "dawn effect" account for an unexplained rise in blood sugar levels during the early morning, which may also influence a child's dreams. Diabetes can have serious effects on the health of your child. So, if she does exhibit these symptoms and you cannot isolate a dietary cause, you should consult a medical professional about the possibility of diabetes.