Researchers have found new evidence to explain the reason why people feel mentally exhausted, as opposed to drowsy, from intense thinking.Their studies, reported in Current Biology, show that when intense cognitive work is prolonged for several hours, it causes potentially toxic by-products to build up in the part of the brain known as the prefrontal cortex. This in turn alters your control over decisions, so you shift toward low-cost actions requiring no effort or waiting as cognitive fatigue sets in, the researchers explain. Researchers used Magnetic Resonance Spectroscopy (MRS) to monitor brain chemistry over the course of a workday. They saw signs of fatigue, including reduced pupil dilation, in those people who are doing hard work.