Stage II sleep. On this transverse montage, there is a K-complex in the fifth second, with its typical broad duration (>500 ms), diphasic morphology, and overriding spindle. There are also abundant spindles before and after.

Stage II sleep. There are typical spindles in the central regions, as well as theta and some delta frequencies. Various amounts of slow are often seen in stage II sleep.

Stage II sleep. K-complex, with its typical characteristics: high amplitude, widespread, broad, diphasic slow transient with overriding spindle. On the longitudinal montage (left), the K-complex appears to be generalized. However, the transverse montage clearly shows that the maximum (phase reversal) is at the midline (Fz and Cz).

Stage II sleep. There is a mixture of spindles (bicentral short-lived rhythmic 14 Hz bursts) and POSTS (positive occipital sharp transients of sleep). POSTS occur in stage I, but the presence of spindles is “diagnostic” of stage II.

Stage II sleep. There is a mixture of spindles POSTS (positive occipital sharp transients of sleep), and spindles (fronto-central short-lived rhythmic 14 Hz bursts).