A sleep spindle is a burst of brain activity visible on an EEG

Electroencephalography

Electroencephalography is the recording of electrical activity along the scalp. EEG measures voltage fluctuations resulting from ionic current flows within the neurons of the brain...

 that occurs during stage 2 sleep. It consists of 12–14 Hz waves that occur for at least 0.5 seconds.

Function

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-complex

K-complex

A K-complex is an electroencephalography waveform that occurs during stage 2 of NREM sleep. It is the "largest event in healthy human EEG". They are more frequent in the first sleep cycles of sleep.K-complexes have two proposed functions...

es they are defining characteristics of, and indicate the onset of, stage 2 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.

During sleep these spindles are seen in the brain as a burst of activity immediately following muscle twitching. Researchers think the brain, particularly in the young, is learning about what nerves control what specific muscles when asleep.

Spindles generated in the thalamus have been shown to aid sleeping in the presence of disruptive external sounds. A correlation has been found between the amount of brainwave activity in the thalamus and a sleepers ability to maintain tranquility.

Sleep spindles result from interactions between cells in the thalamus and the cortex.

Sleep spindle activity has furthermore been found to be associated with the integration of new information into existing knowledge.

During NREM sleep, the brain waves produced by people with schizophrenia

Schizophrenia

Schizophrenia is a mental disorder characterized by a disintegration of thought processes and of emotional responsiveness. It most commonly manifests itself as auditory hallucinations, paranoid or bizarre delusions, or disorganized speech and thinking, and it is accompanied by significant social...

lack the normal pattern of slow and fast spindles.