The stability of the sigma sleep spindle.

The present study closely examined the distribution of sigma sleep spindle activity or six 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.