Brain during NREM sleep. Colour Positron Emission Tomography (PET) scan of the human brain during NREM (non-rapid eye movement) sleep. Colour-coding depicts active cerebral brain areas (red) through to inactive areas (blue). During the NREM sleep phase, the brain is inactive and sleeping deeply, showing similar inactivity when sleep deprived. During the REM phase of dreaming sleep, the brain is more active. 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."