Statistical description and evaluation of the interrelationships of standard sleep variables for normal subjects.

This study explores the relationships between the standard sleep variables, particularly between those of NREM and REM sleep. A total of 399 nights of sleep was recorded in 147 adults who had no known pathology. This amount of data allowed for an accurate description of the generally nonnormal variable distributions and established the relative predominance of the intra- over the interindividual variability. Most correlations between variables were low, showing that there is little redundancy in the choice of variables. The relationships between stage 2 and 4 of NREM and REM sleep and between sleep stages and wakefulness were statistically significant. We found that a short latency of stage 2 predicted a sleep of poorer quality than did a longer latency and confirmed that stage 2 has a central role in transition between stages. Finally, there was an association between the variables describing sleep stability and those describing cyclic organization and sleep efficiency. However, it cannot be determined from these data whether teh relationship is causaL or permissive. In addition, these results suggest that further work on cycle structure is required and that future experiments should incorporate a larger number of observed nights per individual.