WASHINGTON (AFP) - When people do not get enough sleep, they tend to make overly optimistic decisions and may be more prone to risky gambling, US researchers said Tuesday. The study published in the journal Neuroscience provides scientific evidence for what casino managers have long known - that flashing lights and ringing slot machines enourage gamblers to keep going until their money is gone. Scientists used magnetic resonance imagining (MRI) to examine the brains of people who had spent a night of disturbed and shortened sleep compared to their better-rested counterparts. The scans showed increased activity in the parts of the brain that assess positive outcomes, and decreased activity in the areas that process negative outcomes.