No-one knows why we need to sleep, but one theory is that the cortex, the convoluted outer layer of the brain where we do our thinking, needs a complete rest every now and then. New research from J. Christian Gillin and colleagues from the University of California, San Diego hints that it might not be that simple. They have found that the bits of your brain that work hardest while you are awake are not always the parts that need sleep the most. As we all know, after a sleepless night, people generally perform less well.
Reaction times are longer, for example, and short-term memory becomes less efficient.
Received wisdom also says that the shorter and more interesting the task, the easier it is to keep sleepiness at bay and perform almost normally. In normal circumstances, after a good night's sleep, this task activates particular areas within the 'prefrontal' and 'temporal' regions of the cortex. The surprise, however, was how the prefrontal cortex behaved after a sleepless night. This is normally one of the most active areas of the brain when a person is awake, and so could be particularly vulnerable to sleep deprivation. We use our prefrontal cortex all the time to update working memory, including verbal memory, to plan ahead, to decide what to attend to and what to ignore, and to deal with anything new.