A sleep spindle (also referred to as sigma bands or sigma waves) is a sudden burst of oscillatory brain activity that is generated in the reticular nucleus of the thalamus (hyperlink?). This activity burst consists of 12-14-Hz brainwaves that occur for at least 0.5 seconds during stage 2 sleep (hyperlink?). This activity is readily visible on an EEG (Electroencephalogram) machine. Sleep spindles are believed to represent time periods where the brain is inhibiting mental process in order to keep the sleeper quiet and in a tranquil state. These sleep spindles, along with the associated K-complexes(hyperlink?), are characteristics of the onset of stage 2 sleep which typically occur during the early stages of sleep.