Sleep spindles (sometimes referred to as sigma bands or sigma waves) may represent periods where the brain is inhibiting processing to keep the sleeper in a tranquil state. Along with K-complexes they are defining characteristics of NREM(Non-REM) II sleep. Sleep spindles indicate the onset of NREM(Non-REM) II sleep. They are often tapered at both ends and frequently seen over the frontal and central head regions. They may or may not be synchronous, but they should be symmetrical and bilateral.
Sleep spindles result from interactions between cells in the thalamus and the cortex.