STABILITY OF SIGMA SLEEP SPINDLE

The distribution of sigma sleep spindle activity was examined in 6 normal human males who slept undisturbed for approximately 8 h/night for 3 consecutive nights. Sigma spindle activity was monitored by an automatic spindle detector system which performed at 92.5% accuracy when judged against established visual criteria. Individuals differed markedly in total sigma spindle production across nights, but the failure to detect significant differences among nights and the large intra-class correlation suggests noteworthy inter-night stability of the sigma spindle. When sleep onset was used as the temporal origin, sigma sleep activity was essentially flat across the night until the last, REM (rapid eye movement)-dominant, hour which showed a sharp reduction in the number of spindles produced. The sigma spindle density revealed consistent patterns within individual subjects. Intra-stage spindle densities exhibited only minor variations from hour to hour within each night and were stable across nights. As anticipated, the density functions were greatest of stage 2 regardless of night or hours within a night, negligible in stages 1 and REM, and intermediate for stages 3 and 4.