When you lose concentration and "tune out" your brain activates what is called the "default network" – rather like a screen saver of the mind. However, it does so differently if you're sleep deprived compared with if you're not. Ninad Gujar of the University of California, Berkeley, and colleagues used functional magnetic-resonance imaging to observe the workings of the brains of sleep-deprived and well rested people as they pushed a button in response to viewing a picture.
In both cases the "default network" was activated between pictures, but there were significant changes in the activation patterns of sleep-deprived brains. It was possible to gauge with 93% accuracy if a volunteer was sleep deprived or not from the altered activity in only two areas of the brain – both associated with decreased performance on memory tests. Being tired really does appear to have major impacts on the functioning of the brain.