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.

To find out how the sleep-deprived brain differs from normal, the San Diego team used the non-invasive technique of functional magnetic resonance imaging (fMRI) to follow brain activity during a simple verbal recall task. In normal circumstances, after a good night's sleep, this task activates particular areas within the 'prefrontal' and 'temporal' regions of the cortex.

Gillin's group report in Nature1, that after about 35 hours without sleep, people found it harder to recall a short list of spoken words, managing only two or three on average out of a list of five. Activity in the temporal cortex was indeed much less than normal, and in those who performed best, small areas in a different region of the brain -- the 'parietal cortex' -- had become newly active, apparently in an effort to compensate.

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. The San Diego team had expected that its activity would be much reduced, in parallel with the poorer performance in word recall.

But, as always, the brain has surprises in store. Contrary to expectations, the prefrontal cortex became even more active than usual after a sleepless night. Gillin's group speculate that this might reflect the brain's struggle against sleepiness.