Brain during deep sleep. Colour Positron Emission Tomography (PET) scan of the human brain during deep sleep. The colour-coding depicts active cerebral brain areas (yellow) through to inactive areas (purple). The brain is inactive, undergoing a deep sleep during the phase of NREM (non-rapid eye movement) or non-dreaming" sleep. When dreaming, the brain is more active than this. PET scanning shows the metabolic activity of the brain. A radioactive tracer (here, radio-labelled glucose) is injected into the bloodstream 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."