Definition of a sleep spindle

Since the beginning of EEG recordings, different kinds of oscillations were observed in the brain electric activity. Among them special attention was paid to "waxing and waning " wave of frequency around 13 Hz. The first commonly accepted definition of that structure -sleep spindle- was given by Rechtschaffen and Kales (1968):

"The presence of a sleep spindle should not be defined unless it is of at least 0.5sec duration, i.e., one should be able to count 6 or 7 distinct waves within the half-second period. Because the term ``sleep spindle'' has been widely used in sleep research, this term will be retained. The term should be used only to describe activity between 12 and 14 cps."

As the knowledge about the phenomena was increasing, the definition evolved and presently most authors agree with M. Steriade:

Spindles are the epitome of EEG synchronization at sleep onset. This type of oscillation is defined by the association of two distinct rhythms: the waxing and waning spindle waves at 7-14 Hz with sequences lasting for 1 - 2 seconds, and the periodic recurrence of spindle sequences with a slow rhythm of 0.1 -0.2 Hz.

The definition is based on visual analysis of sleep spindles sequences in EEG recordings (in humans) and in recordings from implanted electrodes in animals.
Example of a sleep spindle from human EEG