Brain during sleep deprivation. Coloured Positron Emission Tomography (PET) scan of the human brain during sleep deprivation. Colour-coding depicts active cerebral brain areas (red) through to inactive areas (blue). During sleep deprivation the brain shows general inactivity, similar to the phase of NREM (non-rapid eye movement) deep sleep. Overtiredness leads to sluggish reactions and forgetfulness. PET scanning shows the metabolic activity of the brain. A radioactive tracer (here, radio-labelled glucose) is injected into the blood and absorbed by active tissues of the brain. The PET scanner detects photons emitted by the tracer, to produce a slice" image of the brain."