Brain which is awake. Coloured Positron Emission Tomography (PET) scan of the human brain when it is awake. The colour-coding depicts active cerebral brain areas (red) through to inactive areas (blue). When awake the brain is alert, with high brainwave activity. The awake brain shows activity which resembles the phase of REM (rapid eye movement) sleep when the brain is dreaming. 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."