Sleep spindles appear as a burst of rhythmic activity at the midline and near the vertex in stages 2 and 3 of non–rapid eye movement (NREM) sleep. Their waveform has a characteristic change in amplitude during the burst with lower amplitude at the burst’s beginning and end, and maximal amplitude at about the midpoint. This shape, which is a rising and falling rhythm, lends the pattern’s name spindle because it appears similar to a wound thread or string on a rod. The spindle waveform is not specific to this pattern; the alpha rhythm commonly also has a spindle waveform although the frequency and location differ. Spindles begin and end abruptly and contain a single, unchanging frequency that typically is between 12 and 14 Hz but may be between 11 and 16 Hz (Silber et al., 2007). After early childhood, spindles during light sleep are centered over the vertex with a frequency that is typically 13 to 14 Hz. During deep sleep, they are more anterior with a field that is centered over the parasagittal–midfrontal region and they have a slightly lower frequency that is typically between 11 and 13 Hz (Blume et al., 2002; Niedermeyer, 1999b). Regardless of location and frequency, spindle duration in adults is typically between 0.5 seconds and 1.2 seconds, but it can be up to 2 seconds. The duration can vary considerably within an EEG recording across the spindle occurrences. Overall, spindles after early childhood normally are symmetric along the parasagittal midline, and consistent lateralization indicates abnormality on the side lacking spindles.

Spindles change considerably with age from their first occurrence at about 2 months until about 2 years (Mizrahi, 1996). During infancy, spindles are arciform and unilateral or asynchronous in the frontal regions with an overall symmetry across occurrences. During this time, their durations may be up to 10 seconds, and they can recur as often as every 10 seconds. More commonly, they last 2 to 4 seconds and recur less often. After the age of 1 year, they transition into a sinusoidal, bisynchronous, central, and shorter burst. After the age of 2 years, asynchronous spindles indicate abnormality.

Regardless of age, spindles typically are lower in amplitude than the surrounding background slow activity. Nevertheless, they usually are clearly apparent as the predominant wave in the central region when they occur because they are not superimposed on background slow waves or other activity. However, they sometimes appear fused to an immediately preceding K complex, and K complexes commonly have spindles following them. Spindle amplitude is maximal in adolescence and decreases through adulthood (Gibbs and Gibbs, 1964; Niedermeyer, 1999a; Niedermeyer, 1999b; Principe and Smith, 1982). By late adulthood, they may occur less commonly (Crowley et al., 2002). In contrast to amplitude and number of occurrences within an EEG, their duration and frequency within the burst does not change after early childhood.

Extreme spindles are a rare spindle variant that occurs in EEGs of about 0.05% of children and are associated with cerebral palsy, mental retardation, and encephalitis (Gibbs and Gibbs, 1962; Gibbs and Gibbs, 1964; Heatwole et al., 2005; Niedermeyer, 1999a

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