Sleep is vital for humans. If adults remain awake for longer than usual, the brain responds with an increased need for deep sleep. This is measured in the form of "slow wave activity" using electroencephalography (EEG). In adults, these deep-sleep waves are most pronounced in the prefrontal cortex -- the brain region which plans and controls actions, solves problems and is involved in the working memory.

Source: Developing brain regions in children hardest hit by sleep deprivation