Human non-rapid eye movement stage II sleep spindles are blocked upon spontaneous K-complex coincidence and resume as higher frequency spindles afterwards

The purpose of this study was to investigate a potential relation between the K-complex (KC) and sleep spindles of non-rapid eye movement (NREM) stage II of human sleep. Using 58 electroencephalogram electrodes, plus standard electrooculogram and electromyogram derivations for sleep staging, brain activity during undisturbed whole-night sleep was recorded in six young adults (one of them participated twice). NREM stage II spindles (1256 fast and 345 slow) and 1131 singular generalized KCs were selected from all sleep cycles. The negative peak of the KC, the positive peak of the KC (where applicable), and the prominent negative wave peak of slow and fast spindles were marked as events of reference. Fast Fourier transform-based time-frequency analysis was performed over the marked events, which showed that: (a) fast spindles that happen to coincide with KC are interrupted (100% of 403 cases) and in their place a slower rhythmic oscillation often (80%) appears; and (b) spindles that are usually (72% of 1131) following KCs always have a higher frequency (by ~1Hz) than both the interrupted spindles and the individual fast spindles that are not in any way associated with a KC. This enhancement of spindle frequency could not be correlated to any of the KC parameters studied. The results of this study reveal a consistent interaction between the KC and the sleep spindle during NREM stage II in human sleep.