Brain during REM sleep. Coloured Positron Emission Tomography (PET) scan of the human brain during REM (rapid eye movement) sleep. Colour-coding depicts active cerebral brain areas (red) through to inactive areas (blue). During the REM sleep phase, the brain is active and dreaming, showing similar activity when awake. In the non-REM phase of sleep the brain is in a deeper, less active sleep. PET scanning shows 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."