Not getting enough sleep can harm mental and physical health. In addition to changes in brain development and insulin resistance, studies have also linked sleep deprivation to large increases in weight gain.

Until now no one knew why. But recently scientists discovered that it isn’t just because you are staying awake longer and have more opportunity to eat. Less sleep actually primes the brain to seek out and over-eat sweet and fatty foods.

When deprived of sleep, the brain region associated with reward seeking and addictive behavior undergoes dramatic chemical changes according to a new study. That region, the nucleus accumbens, controls humans' desire to eat food for pleasure, rather than hunger.

The sleep-deprived rats also exhibited enhanced levels of another opioid protein often found at high levels in depressed humans experiencing drug withdrawal.

Eating foods strictly for pleasure is not a new concept for nutritionists or neuroscientists. A prime example is the way a diet rich in carbohydrates can lead to a greater craving for food, much the way a drug addict craves drugs of abuse.

Like workers on overtime or people watching late-night TV or working the night shift, rats in the current study were forced to walk on a treadmill during the hours they would normally be sleeping. The sleep-deprived rats were compared with a set of animals that obtained the same amount of exercise (during the day) and another group that were deprived of food.

After brain scans were performed on all three groups, the researchers compared results. The sleep-deprived rats demonstrated increased activity of the gene that produces an opioid peptide called proenkephalin (PENK), which is enriched in the nucleus accumbens region. Proenkephalin is normally found in the brain, where it activates the same receptors as morphine or heroin.

In contrast, the PENK gene remained at normal levels in the brains of rats that were not sleep-deprived, even if they received the same amount of exercise or were kept on a reduced diet.

The enhanced amounts of the PENK opioid are thought to magnify the pleasures of eating junk food. Under normal conditions, the extra opioid produced by the gene declines rapidly immediately after eating, telling the brain to stop eating beyond any nutritional need.

With the sleep-deprived rats, however, the PENK gene was stuck in the "on" position. In addition to containing high amounts of the proenkephalin gene, the sleep-deprived rats also exhibited enhanced levels of another opioid protein. This chemical, prodynorphin, is often high in depressed humans experiencing drug withdrawal – a condition that has been likened to yo-yo dieting patterns because they produce similar chemical changes in the brain.

“If you want to lose weight, it helps to get enough sleep,” Brian Baldo, lead author of the study, said in a statement.

Dr. Baldo's laboratory at the University of Wisconsin-Madison studies the neural basis of eating disorders and addiction. The researchers plan to use findings from the present study to determine possible targets for drugs that could control food cravings, such as molecules within the opioid system.

The study is published online in advance of publication in the journal, Neuropsychopharmacology.