We've probably all done it at one time or another, you’re sat on the sofa at night and you’ve already eaten a few rows of chocolate. You’re starting to feel full, but for some reason you feel compelled to finish the rest of the bar off and you’ve no idea why. Well now researchers have found new evidence to explain why many of us are unable to resist a good chocolate every now and then. The urge to overeat such deliciously sweet and fatty treats, traces to an unexpected part of the brain and its production of natural opium like chemical. The area, part of a larger brain region called the striatum, had previously been primarily linked to the control of physical movement. The new research published this week in the journal, Current Biology and carried out by Alexandra DiFeliceantonio of the University of Michigan, tested this area of the brain out on rats. DiFeliceantonio's team made the discovery by giving rats an artificial boost, with a drug delivered straight to a brain region called the neostriatum. Those animals gorged themselves on more than twice the number of M 'n' M chocolates than they would otherwise have eaten. “This means that the brain has more extensive systems to make individuals want to over consume rewards than previously thought. It may be one reason why over consumption is a problem today,” said DiFeliceantonio. The scientists, from the University of Michigan, studied the area because there were some hints that it might be involved in reward seeking behavior, specifically in encoding just how rewarding something should feel. So the scientists decided to see what would happen if they infused the area with a synthetic version of the endorphin enkephalin, which acts naturally in the region, while letting rats eat as much as they wanted of M&M chocolates. When the researchers infused that specific area with enkephalin, whose most well known function is as a pain relieving peptide, the rats suddenly upped their intake of chocolate by more than double. And when they took another group of rats and measured the levels of naturally occurring enkephalin in the area, they found that M&M consumption led to an immediate increase in enkephalin. And the more enkephalin they detected in the region, the faster the rats ate. The study found that enkephalin binds to molecular receptors in the brain, sensitive to opiate chemicals to reduce pain and produce pleasurable feelings. The extraordinary findings showed comparisons between drug addicts and obese people. DiFeliceantonio said, “The same brain area tested here is active when obese people see foods and when drug addicts see drug scenes. It seems likely that our enkephalin findings in rats mean that this neurotransmitter may drive some forms of over consumption and addiction in people,” The researchers now hope to unravel a related phenomenon that some of us might wish we could do more to control, what happens in our brains when we pass by our favorite fast food restaurant and feel that sudden desire to stop. In their paper, the scientists concluded, “Opioid circuitry could in this way participate in normal motivations and perhaps even in generating intense pathological levels of motivation, to over consume reward in binge eating disorders, drug addiction and related compulsive pursuits.”